I haven't picked one up, yet. Then again, it's still early (my classes just started last week). Most of the students from last year have told me that it's not that helpful. The study guides will not emphasize those things my professors consider important, and they will likely emphasize things that won't even be tested at all. Also, some of my professors have really strange idiosyncrasies (my contracts prof is reputed to be like this), where when it comes to analyzing a legal problem (say, restitution/unjust enrichment, on which this prof is a leading authority), it's his way or the highway. A study guide would probably help more than hurt with such a prof.
Bottom line: I don't know whether I'll turn to them or not. It doesn't seem warranted at this point. We'll se how I feel in late October/early November.