I just recently purchased an n900 myself. By far, the most significant distinguishing feature of this machine is not the hardware, but rather the operating system; Maemo is an open source, Linux-based OS, and fully embraces the open source community. As such, instead of billions of tiny apps, the n900 is capable of running standard, full-featured Linux applications, making it much more of a palm-top computer than anything else in this form factor.

Browsing the internet on this device is much more like browsing on a tiny laptop computer than on a smartphone. For one thing, Flash is supported on this phone, so a huge number of websites that don't play well with other smartphones work fine on the n900. And the screen's resolution is high enough that many desktop-oriented web pages will fit the width of the screen without resizing.

One difference of this machine with other recent phones is that it has a resistive, rather than a capacitive touchscreen. A downside of this is that multi-touch operations are not possible; you can't do the "pinch-to-zoom" thing. On the other hand, you can use a stylus with this sort of screen, which makes fine interaction with windows and icons much easier. It'd be nice to be able to do both, but I tend to prefer the stylus over multi-finger use.

I do like the keyboard. I don't have much experience with other cellphone keyboards, though.

Is it outdated? Certainly, new hardware appears every year, and there are phones out there with slightly faster CPUs or more memory or whatever. Rumors of the n900 being discontinued and its successor nearing release are swirling about. The problem is, whatever you choose, there will always be newer and better devices just around the corner. For me, the n900 does everything I want, and no other device supports Linux the way it does, so I had no qualms about choosing it.

On the downside, this phone is nowhere near as popular as the iPhone or the various Android phones, and "app" support is therefore much more limited. If you're looking for a phone that has "an app for that", the other choices are much more likely to suit your use.

In short, I'd go for this phone if you want the ability to carry around a tiny device that can do almost anything a Linux computer can do; I'd go for a different one if you just want to carry around a phone that also has some fun apps to play with.