It's always difficult to precisely measure where and when there's a performance improvement.
FWIW, with Lightroom 4.2 I can safely recommend it to anyone, where with 4.0 and partially 4.1 I would have advised to try the trial first.
Yep, it's really hard to judge "snappiness" accurately but I found that going from 4.1 to 4.2RC did improve UI responsiveness. Updating from RC to the release version of Lightroom 4.2 did bring no seat-of-the-pant speed.