I've also been running WM5 for a few weeks and am relatively happy with it. However, for me there is one main "show stopper" that will probably cause me to go back 2003SE: NetFront v3.2 doesn't work. IMHO, IE is still crap in WM5, so I've gotta have an add-on browser to get anything serious done (e.g., OWA).
Besides the NetFront problem, there are also a number of annoyances that are keeping me teetering:
1. WiFi and BT are flakey. WiFi sometimes shuts off while connected, BT comes on randomly, etc. I'm getting kind of sick of this. Not to mention the UI for these seems really dumbed down in WM5. I don't feel like I have control...
WiFi finder tools like Pocket Winc don't seem to work anymore.
2. Remote display from MS doesn't work. I used to use this A LOT and I'm missing it.
3. Syncing via AS4.0 or 4.1 seems really slow compared with before. Also, the AS client on the mobile side seems to be always running and I can't get rid of it.
4. AvantGo doesn't seem to want to sync over USB (works fine over WiFi & GPRS).
5. I can't get FeederReader to work. Crashes on startup.
6. I can't get my Bluetooth keyboard to work. s/w installs, but won't startup.
7. I'm not sure I like how the reminders and other messages pop in from the bottom. Maybe this is configurable?
8. The battery meter in MagicButton doesn't work.
I could go on, but I think I'd be getting nitpicky. On the positive side:
1. Phone app is way better (faster, more functionality, better organization, etc.).
2. UI is slicker (i.e., more eye candy).
3. Can set the GPRS class (8 or 10).
4. BT static virtually gone (I'm using a Moto HS810).
5. ActiveSync over BT is finally easy to set up and get working!
6. Can have multiple AS partnerships.
Bottom line is I like it and want to stick with it, but I need a better browser, FeederReader, remote display, and my WiFi tools. I'll probably continue to experiment and follow what's happening here and on xda-developers. But if it wasn't such a hassle to go back to 2003SE, I probably already would have.
Hope the above helps some of you who are considering making the leap.
Paul