I went from a new phone to the N900, and now to a SGS2.

The Good:
Games on the N900 are awesome, doom, Openarena, Atomic Tanks, etc.

Keyboard: Awesome, loved being able to use it

Storage: Large drive

FM Transmitter: Perfect for playing music in any car

IR Port: Fun to screw around with tv's

Linux: Scripts, Python and other support

Battery could last for 2-3 days

bleeding edge wifi drivers for packet injection

The Bad:

MMS is iffy at best

Modrana is the best GPS program for navigation but still force closes

No pinch to zoom on the screen

Small screen makes it harder to see things

Video playback is rough on higher quality stuff

So for those reasons I went to the SGS 2. It also supports packet injections, I lost the FM transmitter, IR port, hardware keyboard, and mult-day battery.... but I gained Google Maps, bigger screen, Swype, MMS, gestures for zoom etc and video playback