I didn't take Kaplan for LSAT, but I have a lot of friends who did both Kaplan and BarBri for last July's bar exam. The verdict, at least in IL, is that BarBri didn't prepare people adequately for the state-specific portion, which is pretty ridiculous seeing as the state portion has always been BarBri's bread and butter.
I heard that Kaplan is doing full service in some places now, and from what I heard, it sounds okay. For one thing, it's cheaper to do it online than to go to lectures, and BarBri makes you pay more to listen to lectures on an old iPod that you have to give back. BarBri also makes you pay extra for more graded essays, and I heard that Kaplan has unlimited grading. I think Kaplan has a money back guarantee, too.
I took the MPRE over the summer (just found out I passed!) and I did both the free Kaplan course and the BarBri course that you had to have paid $250 to take, and I liked Kaplan better because it had a lot of practice questions. BarBri gives you a book and a "computer option" which is really all the same questions from the book, but on the computer. I thought that was pretty lame.