Got around to installing LR4 I had bought couple days ago when it was released. Unfortunately, it's less responsive than LR3, not annoyingly so, but just enough to be noticeable (FWIW, I'm using early 2011 15" MBP w/ 2.3 GHz i7 & 8GB of RAM). Oh, and now my plug-ins don't work.
This is what happens when I don't wait to get a software until after a patch or two like I usually do.