I have had my Nokia N900 for over 4 months now since it came out and I can safely say I have mixed feelings about this phone, or as Nokia like to call it, a "mobile computer".
Talking about the built in features:
Web Browser - Very fast, with built in flash, best mobile browser I've seen.
Communications - The talk feature works like any other phone, however the texting and Instant Messaging has been implemented really well, meaning you can communicate with somebody through many different networks through one window.
Performance - N900 By far is the most powerful handheld device I've seen, I managed to run 40 applications varying from video, music, games, gps, all at once and the phone did not show any fatigue.
Navigation - The built in ovi maps application is currently poor, it doesn't run at good speeds and has a problem with mapping routes, however third party community navigation apps work really well.

About the phones diversification:
First of all I was misled into thinking this would be an easy to use device, such as an apple iphone which I has previously had. For e.g. on the iphone I would open the appstore find and app and do a one click download; However on the Nokia N900 it is not as simple as the phone is "open source". You have to go through many small procedures, like adding repositories. This is a good and a bad thing. If you are a simple user and like to just get up and running with a phone, this phone is not for you. However with time, patience and some knowledge in this area the N900 is a far greater device than any other.
Within 2 weeks of me reading tutorials and getting to know how the phone works I had all my old retro console games working and innovative useful apps and even now the open source developers are coming out with new and exciting features for this phone. This is all because the phone relies on community development. It means that applications and features for the phone can be unordinary and tailored to your liking. Also applications can be made which would not usually be allowed on other handsets. Community development also means applications are released at any time, and the quality of them can be inconsistent.
My final rating for this device would be 4.5/5.
Loosing 0.5 only because it is not a simple phone to use.