I use LR6.4 all the time (Rarely use CS6) - I didn't like LR before 4 (No idea why, just didn't get it I think!).
From memory there are a lot of improvements in 6 from 4 regarding the processing engine for one area.
You can always install the trial and if you don't like it (That would genuinely surprise me!) un-install and go back to LR4.