Accepted.com actually has a great PDF out there called "LSAT Prep Companies: In Their Own Words," that does a great job of presenting a thorough look at the bigger prep companies out there: Altas (although I believe they've been bought out?), Princeton Review, Get Prepped, PowerScore, Kaplan, Knewton, Inspirica (I haven't heard of them), Test Prep New York (haven't heard of them either), and Blueprint. Linda Abraham, who runs Accepted, is a solid authority on things LSAT and law school, so I would definitely give this PDF a read if you're just starting your LSAT prep search:
http://www.accepted.com/law/LSATPrep.pdf