Veeeryyy customizable for starters...
A good lil' hacking machine...right up to penetration testing tools.
The ability to run a good list of alternate operating systems and desktops...
Debian...(yup) , chroot easy debian (much easier), android for Nokia Internet Tablets (Nitdroid) dosbox, openbox, lxde, kde, E17 enlightenment , palm os (garnet), mac (basilisk), windows (95 and something else ...can't remem. ) mer, backtrack (I think...can't remem) ....Ubuntu Jaunty .....some run great some need more work, updating or fixing, some were done simply as proof of concept that it could be done I swear ..pretty cool all around...I'm sure I'm forgetting some
Clone your onboard os to your sd cards...back up your whole system and store it on your pc or wherever.lotsa cool stuff.

it has 2 sd card slots
great for multibooting multiple os's from your sd cards...
this is the storage cap I got a couple of mine to...

http://talk.maemo.org/showthread.php?t=90634

all around a pretty great device...I could go on ...but there are a ton of posts extolling it's virtues. the n810 has the physic backlit keyboard....built nice and slimmer....but has only one card slot for mini/ micro sd....good device....but I love the n800 for the beast it is and what it can do.