Well, I've been using beta 1 since it came out and since got a dev account too(used shift-update for beta 1 so I didn't restore from iOS5). Looking forward for beta 2 tonight, restore time for sure.
-auto-brightness is really trying to keep the screen very bright all the time
-auto-correct is even more retarded than before, changing correct words to wrong ones and slightly misspelled to completely different. Sometimes keyboard shortcuts require a 2nd try
-iPad 3 seems to keep cooler, it's very random though. It sometimes heats up for no reason and sometimes keeps quite cool during 3D gaming
-battery life is good(I've got 1d6h standby and 11h usage on iPad 3). It's nice that they finally added the usage times on iPad too.
-photos is buggy, I've saved photos and yet it doesn't show them or upload to photo stream. The photos I've saved are actually saved on the device, it's just not loading them.
-iMessage isn't changed IMO, still inconsistent. Some new bugs include missing "read notifications" and incredibly slow sending.
-Safari is WAAAAY more stable on iPad 3. It used to crash almost every time I switched from Messages back to it, now it never happens. While it doesn't crash as often, it freezes on some websites which means it'll still crash and waste 1-2 minutes of your time while doing so.
-the changing status bar colors need some work, I like the idea but it doesn't look good yet. Black status bar with current safari design on iPhone also looks weird. A silver safari like on iPad would fix this.
-iPad sometimes decides not to rotate without a reboot(also happens on iOS 5)
-multitasking gestures get randomly broken until exiting current app.