My reasons for this have nothing to do with brand preference or personal bias; they are entirely logical and reasonable. The N900 is the only smartphone produced by a major manufacturer that comes with a real command line right out of the box. You don't need to install any apps. You don't need to jailbreak the phone. You don't even need to change any settings. When you first power on the N900, there is a full Linux command line available from the list of installed programs. There is no other device I know of on the market today that does this. This alone makes the N900 the only smartphone in the world worth owning.
Lest there be any doubt, be assured that the Nokia N900 is a true pocket computer running a real Linux distribution. The N900 uses Maemo 5; it is the first and the last device to do so. Previous versions of Maemo were designed for Nokia "Internet tablets" which used Wi-Fi and were not cellphones because they did not connect to cellular networks. After the N900, Maemo mutated into MeeGo, which was subsequently killed off after also only being used in a single device (namely, the Nokia N9, which is much worse than the N900 because, among other shortcomings, it lacks a hardware keyboard). Maemo 5 is a true Debian Linux distribution, and yes, it can install .dpkg files natively without any modification to the stock configuration of the phone. Name another smartphone on the market that can do that. Go ahead, name one. Name another phone that can use the Linux binary repositories to download applications instead of the phone manufacturer's own proprietary "App store." (Nokia also had something like this in the form of Ovi, which the N900 comes with a link to, but you are free to ignore that link and use the Debian repositories to download your applications.)
The N900 itself is pretty decent as a phone: As I mentioned, it uses a physical keyboard, which is an absolute requirement for anyone who knows what they're talking about. It also uses a resistive touchscreen instead of a capacitive touchscreen, a precious rarity in the smartphone world today since it means that you can use a stylus on the screen instead of your fingers, which offers much greater precision in tapping specific locations on the screen. The N900 also has two speakers built in, one on each side, which allows true stereo sound--something quite special in a smartphone. However, as good as the N900's hardware is, the real star of the show is the operating system, Maemo 5. Most enthusiasts agree that Maemo 5 is simply the most hackable major smartphone OS in the world; it consists primarily of open-source components, and because the N900 is a carrier-agnostic phone (it is sold unlocked instead of as part of a carrier-subsidized package), it is not cluttered with efforts by wireless carriers to distinguish their offerings. What you get with the N900 is simply the most open, most customizable phone available from any major smartphone manufacturer.
How very unfortunate, then, that Nokia promptly decided to kill off Maemo (and MeeGo) and go with Microsoft as the future of its smartphone business. Nokia is no longer developing Maemo, and will never release another Maemo or MeeGo device. In light of this, I reached the conclusion that the N900 is the last smartphone. Since there will be nothing in the future as good as it, it will be the world's last smartphone worth using. 10 years from now, the Nokia N900 will still be the only smartphone worth owning or using. 1,000 years from now, the Nokia N900 will still be the only smartphone worth owning or using. Period. The smartphone industry's history has been written. There will be no updates to it anymore.
Unfortunately, the problem with any portable device such as the Nokia N900 that limits its longevity is the battery. Almost everything else is theoretically replaceable; the N900's physical keyboard has mechanical keycaps which will eventually wear out, and the screen may also eventually die over time, but the battery is the single most limiting factor in the lifetime of any portable computer. Like most smartphones, the N900 uses the manufacturer's proprietary battery model, which means that when they stop selling them, you'll stop being able to get batteries for your device. In this case, the N900 uses the Nokia BL-5J battery, which is still fairly widely-available for just a few dollars per battery, partly because it is also used in several other models of Nokia devices. Unfortunately, I expect that eventually Nokia will stop manufacturing the BL-5J. When that happens, the N900's days will be truly numbered; eventually, there will come a time when you won't be able to turn it on.
There are other problems facing the N900, as well. Nokia is cutting off their funding for the maemo.org website, which includes talk.maemo.org (regularly abbreviated by users to "tmo"), the official Maemo forums. It also notably includes repository.maemo.org, the Maemo repository of applications. This went offline recently as a result of the need to shift this resource away from Nokia's own hosting; this infrastructure is now being handled completely by hobbyists, people who are supporting the N900 because it's the world's only smartphone, and because Nokia is no longer supporting it. If Maemo's official repository disappears, it is possible to use packages.debian.org in the N900's application manager to download everything, but the Debian package lists are enormous and would take a long time to transmit to the N900 over a cellular link. Not to mention the fact that most of the programs there are intended for desktop PCs, and it was nice to have a dedicated application repository that was specifically for Maemo devices. The torch has been taken up by the Hildon Foundation (named, interestingly enough, not after Maemo the operating system, but rather Hildon, the application framework used by several devices including the N900), but the Hildon Foundation appears to be strapped for funding, declaring that they may be unable to keep the site going without regular donations to the tune of about $1,000USD per month, which seems like an astonishingly low number given how many Maemo users there are (or I thought there are) worldwide. If they can't raise that much in funding each month, then Maemo's userbase is truly dying, and eventually there will no longer be even an unofficial repository of Maemo applications online.
As time goes by, little details like this are going to start working against the N900, and it's going to become less usable. Unfortunately, I see no way around this, and although I've been paying attention to the rest of the smartphone market, I see no usable alternatives there.
First off, there's Android. This would be my first choice simply because it is supposed to be the smartphone OS for "power users" who actually know what they're doing. Unfortunately, the problem with Android is that it's terribly fragmented; because each carrier customizes the Android base install for each model of phone, there is far too much cross-incompatibility across different versions and builds of Android. It's a fundamentally decent operating system, but I dislike the fact that Google simply took Linux, rebranded it, and acted like they made their own operating system. They didn't; all the difficult details of the kernel were taken care of by Linux. Android is just like Apple's OS X: A real operating system broken by a bloated GUI and proprietary tweaks. Absolutely broken.
Speaking of Apple, my second choice for a smartphone would naturally be an iPhone. Although I abhor Macs and believe that anyone who uses a Mac should be put to death unless they use it exclusively for media work (graphics, video, and audio editing), I have to admit that the iPhone is a decent little product as a phone. However, I don't use my smartphone as just a phone; in fact, calling people is the least important functionality it offers. I bought a smartphone to use it as a true computer, something to play adventure games in DOSBox on while I'm riding public transit. That is what a smartphone is for. Just like OS X, iOS is Apple's effort to take someone else's work and rebrand it to make it look like they did something innovative with it when all they did is put a pretty interface on it. If I just needed a phone, I could use an iPhone, but honestly, if I really just needed a phone, I would probably just buy a cheap "dumb phone" for $10, one without the ability to install applications or take photos. The iPhone is a decent piece of work in many respects, but I simply cannot tolerate Apple's efforts to control the platform and what users can do with it. I'm not going to buy a device to have the device manufacturer tell me what I can do with it. That's the wrong approach to smartphone use. Absolutely inappropriate.
Then there's Windows Phone. Yeah, I don't think I need to say too much about this one. (Hint: Does it come with a real command-line interface built in?) (Another hint: No.) The same goes for BlackBerry or anything else RIM will put out. I would rather die than support RIM. Microsoft and RIM are giving each other pretty good competition to see who can be the most wrong-minded smartphone vendor in the world; right now it's a dead heat as far as I can tell, but either way, neither of these companies deserve to have anyone buy their products just on principle. Absolutely not.
So that's basically it. Since Nokia also killed off Symbian--until recently by far the world's most popular smartphone OS--we're basically left in a world without smartphones. There is no smartphone in the world worth buying. None whatsoever. Just as I've written in the past about the desktop computer industry, noting that the industry has been essentially eliminated (see Linux: Total loss and They sure want people to stop using computers), we've reached the point where the smartphone industry is dead.
Will I ever buy another device like this? I've been looking at the OpenPandora, which is a pretty decent little portable computer which has the benefit of being (as the name suggests) entirely open in terms of both hardware and software. This definitely appeals to me; it has built-in Wi-Fi, so it could replace a smartphone in places where Wi-Fi is available. Sadly, I do not live in such a place, so the OpenPandora, while a fantastic device, is not a true smartphone replacement since it cannot connect to GSM cellular networks. If it could, it would probably regain the crown as the only smartphone in the world worth owning.
At the present time, however, I'm stuck with the N900, which is a fantastic phone that I'm very happy to own and use on a daily basis, but which I realize I'll probably have to part with someday, most likely for lack of battery viability. When the N900 dies, Maemo will die with it. Tread softly, friends: Maemo, one of the world's most important global visionaries, is slowly dying.