I'm coming from a series of Palm Treos (600>650>680), which is IMHO still hard to beat as an OS for a PDA.
By which, I mean for holding names and addresses, keeping a schedule and diary, securely holding passwords and access codes, and syncing everything to the PC reliably and without hassle,

I made a short trip into WM6 (HTC TyTN2) for a built in GPS and an OS I could write reasonably fast software for. It was hateful, in the end I threw the thing at the wall and dug out my Treo680.

I'm hoping that the N900 can be the best of both - decent OS, customisable/programmable, and with up to date hardware capability.

Oh, and a decent web browser will be a nice change!