I took the Kaplan course and it was way worth it for me in terms of general review of the sciences. I finished the majority of those science classes a significant time before taking the test, so I had forgotten a lot of the details. Kaplan was great for directing my studying and showing me what I needed to know and didn't need to know. Definitely don't rely on Kaplan for all of your studying though. I also really liked the syllabus Kaplan lays out for you - it really helped me in terms of planning out my studying (otherwise I would have been so overwhelmed and wouldn't have known where to start). Kaplan is also great for test-taking tips in terms of time-management, tricks for the PAT, and ways to approach certain questions.
In terms of the practice tests, Kaplan has 5 full-length exams. I would say that they are pretty realistic with respect to the actual DAT, but their PAT section is slightly easier (especially in angle-ranking!). What is great about the Kaplan exams, though, is that they give you a detailed score report and answer explanations that can help you determine your strengths and weaknesses so you can better focus your studying using their material and additional outside resources.
I highly recommend taking a Kaplan course, it was definitely worth my money. In addition to Kaplan, I also used the DAT Destroyer Set (which includes the Destroyer (covering Bio, GC, OC, QR), OC Odyssey, and Math Destroyer). I used all three of these extensively, especially focusing on Odyssey and Math Destroyer because those were my weakest sections. I HIGHLY recommend Crack the DAT for the PAT section and for the QR section. CDP is very similar to the actual DAT in difficulty and may even be a bit harder for some sections.
I haven't heard much about Princeton Review, but from what I have heard, Kaplan seems better. I think PR is stronger on exams like the SAT/ACT, while Kaplan is stronger on graduate exams such as the DAT, MCAT, LSAT, GRE, etc.