How is iOS7 on the 3rd iPad (the first one with a retina screen)?
It's fine. I compared my 3 to a friend's Mini and my father's iPad 4 with iOS 7 and the difference in performance between the models is the same as the one from iOS 6. Many in the thread reported getting normal performance on an iPad 3 after the update, no keyboard issues, no chugging. I honestly don't know what's up with people getting trouble on their third gen iPads but I guess you better not update if you're worried.
Some things I thought wouldn't be on an iPad 4 are present on it too. Changing wallpapers lags and freezes the iPad for a few seconds on all models. The slightly jerky animation when you use gestures to go back to the home screen is also on all models (if you press the home button it's a smoother one but not as speedy).
For the record, I've also been using the update on a 4S. Performance-wise iOS 7 feels less polished than 6 at spots and, even though I'm enjoying the update and it doesn't give me any problems on my daily usage, I'm looking forward to them tweaking those bits in the future.