I have the "sluggish" problem that many others have stated. Also, the battery drains significantly faster than with GB and I think this is due to how ICS handles backround data, syncing, and wifi. I'm sure there's a benificial reason for the power usage/battery drain tradeoff.
A couple things I noticed:
1. Even with Auto Sync turned OFF and mail and weather set to 'manual', I still recieve e-mail, weather, and app updates and can still access Google Play.. where as in GB I had to turn ON auto sync to access Google Play. ..So I'm not sure exactly whats going on in the backround.
2. WiFi is constantly scanning and doesnt stay connected sometimes. I work 12 hour shifts and in GB, I always connected to wifi at work. In ICS, it seems to have trouble staying connected. When I get home, battery is at 20% less than it was in GB.
3. There are alot of cool features in ICS but for some reason I can't access some of them. There is an 'inverted screen' option in Web settings that changes the backround to black and text to white. This makes it easy on the eyes in a dark setting. I can get to this menu but the option to select it is greyed-out.
4. An option that- I think would save battery- is to disable wifi scanning, which can be found in the 'advanced' menu under wifi settings. But this too is greyed-out unless wifi is on (and why would I disable it while I'm using it?) ...It almost seems like I have a "restricted" form of ICS lol.
5. Not sure if this option was here before ICS, but in settings-> privacy, there's an option to 'back up my settings' that is checked by defaut. I noticed this after I did a factory reset and found that everything was still on my phone.. even the wallpaper I was using before the reset.
With the wifi, battery, and auto sync issues aside I am generally happy with ICS. Just think that its a little too much and a little too late for the Thunderbolt. If anything, its something new to play around with till I upgrade in 2 months..