I set up a LR4 Test folder and I've dumped in a couple of hundred images to test it out with, that way it is separate and apart from my LR3 catalogue and images. So far I really like it, so much so I don't like going back to LR3 to work on my non-test images. The replacement of recovery, fill light, and brightness, with highlights, shadows, and whites is a big improvement I think. As it seems to do a pretty good job of identifying both highlights and shadows and making adjustments to those alone. And maybe I've been missing something, but I've always found fill light to be next to useless, but not so with the new highlights feature. They've also made some good improvements to the adjustment brush. I'll certainly be upgrading.