Before I get to those enlightening First Impressions, a couple of notes about my upgrade experiences.
The download (247Mb) took longer than I'd expected, so I went to bed. At 3am, having visited the bathroom, I stopped at the desktop, found the download complete, and triggered the update, then went back to bed. At 3:40, unable to sleep, I checked on the phone to find that it hadn't charged during the previous five hours (some weird USB conflict or problem) and the battery had expired halfway through the update. So, charge the phone (and install iTunes 9.0) then try again.
Two more failed update attempts, and I'm getting a little testy. At about 4:20 I restarted the PC and, at last, the update went through. Back to bed.
At 6:30, on my way to work, I stopped to collect the phone expecting to find it fully synced and ready to go. Instead, nothing was synced except the basic system settings (so I could get email and contacts) and after fifteen minutes of fiddling I gave up and left for work without Apps, music, movies or podcasts. Not ideal.
Now, as I sit on the train, 3.1 definitely feels snappier than 3.0, but I can't really be sure without testing a few apps. It's surprising just how much additional functionality the third-party apps add, and how far the iPhone experience is degraded when they're suddenly unavailable; I really miss them.