One thing that Kaplan DOES offer is structure and a game plan for those that don't have one. While I can't offer any conclusive argument about the level of improvement Kaplan's students see, I can offer my own experience... which is getting a lowly 155 on the diagnostic and not seeing ANY improvement at all during the 14 week class (in class tests were 152/157/155). However, after postponing the LSAT to take advantage of Kaplan's 'Higher Score Guarentee', I saw a significant improvement in not only my in class tests but also my home study. Unfortuantely, as a prior poster indicated, my actual test day score was anywhere from 3 to 8 points lower than my practice tests.
What does that really mean? Well, probably a few things...
1) You only get out of Kaplan what you put it (first time around I was in some heavy classes while working FT, whereas the second time around was during the summer)
2) Scores earned in the optimal low-stress environment of a Kaplan center may be a bit better than what actually occurs during a real LSAT.
3) Kaplan offers some guidance to the uniformed but as a class marketed to the masses, it should not be relied on for someone with T-14 aspirations.