I use the Easy Debian on my N810 with great success and yes, both aptitude and synaptic work. I've gotten software like Scribus to run in Easy Debian. If it'll run on the N810 it'll run on the N900, the hardware is better. Since Easy Debian provides it's own complete debian base it doesn't wholly matter which Maemo you're running it on, it's all a chroot.

The only limitation I was aware of with Maemo 5 was qole was trying to figure out the separate X window to run apps in a more "native" setting rather than squishing it into Maemo's screen. I think he worked that out, however - you can follow the Freemantle Easy Debian disucssion here.

I agree you are leaving end-user territory.. but don't think you're going to totally fubar your device by using Easy Debian (especially if you accept defaults and use the image file, not try creating your own partitions/extracting the file yourself.) I say if your interested in Debian on an N900 - give it a shot.