I have the LR 4.1RC installed on my MacPro 8-core, 32GB RAM, Lion OS, SSD boot drive and RAID0 for images and, sadly, the program is still very sluggish. I get the spinning beach ball a lot even when doing basic functions like cropping. Bummer!
I hope that a real fix is forthcoming.
Other than this, I love the new feature set. LR4 took me off the fence from considering switching to Aperture, although I do love the way Aperture integrates with the Mac OS. The tools in LR are just so much better IMHO, particularly, gradient, profiles and lens corrections to name a few.