I really like it! I think for anyone wanting to upgrade an iPad 2, the improvements are too subtle to warrant the cost, but for a first time tablet owner, it is incredible! I too have never heard of a over-heating issue, but then again, I have always used the iPad 3 in a Targus Slim Case, so I might just not have noticed it. That said, I have never noticed a decline in performance either.
I guess the one negative thing I could say, is that the battery takes FOREVER to charge, around 7ish hours for a completely dead battery. The new retina display uses a ton more power (and even more if you use 4G), so to keep the longevity of the battery, they had to double the battery's wattage or something, making it take much longer to charge. I just have to remember to plug it in at night. Fortunately, the battery does last all day, so I'm always good, just wish I wasn't screwed if I had forgotten to plug it in!