So I'm a week or so in on the n900 now and still liking it. I'm still fighting with sync: the Google sync via Exchange is very very flaky and seems to fall over as soon as it actually has some changed data to sync, so right now I have no good sync methods (how is everyone else doing OTA sync with n900? Please don't tell me to install Funambol, I really do not trust gigantic blobs of Java on my servers.)
Aside from that, though, I'm enjoying it. It really does seem to work well to treat it as a tiny laptop in many ways; I've got it set up with an ssh server and client, now, so I can ssh in and out of it to my other systems. It's nice to just flip between IRC, a browser, email and my password app, just like I do on my main systems. xchat runs very smoothly on it even though the interface isn't perfect; I find myself jumping on IRC on train rides just to see if anyone's waiting on a reply from me for anything. Wouldn't ever have bothered doing that with my WinMo phone as its IRC client was just too slow and clunky.
One really cool thing is the excellent VOIP integration. It's very easy to set the phone up to log in to SIP accounts - like Fedora Talk - and the functionality is very well worked in to the phone app: you can enter SIP addresses for your contacts and call them from the phone book, you can route PSTN calls over one of your SIP accounts, and when someone calls you via SIP it works exactly like a regular phone call. I tested it with Jared Smith and it worked great. So I'll plan to leave my phone signed into Fedora Talk pretty much all the time. So if you want to call me, get a Fedora Talk account, it's all free 'n' stuff!
It also has similarly awesome integration for Skype, though I'm not planning to leave that logged in much due to Skype's evil P2P system eating your bandwidth if it nominates you as a super-node or whatever they call it.
I found the Community SSU, which is a sorta service pack containing updated and patched versions of a few components, including the desktop but more importantly (for me) a newer Modest package with the patch for proper replying (i.e. with a proper attribution line, quoted original message, and THEN your signature) incorporated. Finally, the email is actually useful for something. I wish they'd patch for multiple sender identities too, but can't have everything.
I also found that using Google Maps Mobile through the browser works as a pretty decent Maps application replacement - it has GPS and transit routing and everything. Zooming out is the only thing you can't really do (well, you can do it by editing the URL). But it's definitely usable.
So, aside from the syncing issue, I'm definitely liking it so far, not feeling any inclination to switch back to the old phone.