I took Kaplan and it improved my total score four points in three months, with an average of about ten hours per week outside of class. I'm sure Princeton is pretty much the same. I would definitely cough up the cash for one of these courses! They'll tell you exactly what to study and give you good structure. A lot of the classroom crap is a waste of time, but they make a ton of resources available to you, like practice tests. The secret to scoring 20+ on the DAT is to just put in the time. If you've had all of the required science courses, and put in time reviewing everything Kaplan tells you to, you should be able to get 20+.