Go to
Rabbit's Reviews and search for the site(s) you are curious about, then scroll to the bottom of the reviews. Rabbit allows user comments and they can give you a pretty good insight, especially with trial memberships. Disgruntled members are more than happy to report back if a trial membership is really limited or shitty, or if going for a full membership isn't worth it. Presume that the really good user reviews are by site owners/employees and that some of the really bad reviews are from cheap fucks who can't be pleased. The truth is somewhere in the middle. There's usually a user score in the right column of the review too, which will almost always be way below the reviewer's score.
Actually, I've discovered that Rabbit's is a nifty tool for any webmaster promoting revshare sites, trial or not, because you can see approximately how much content the site has, its quality, and other measures by which you can conclude whether or not the site will retain members. You'd be shocked by the amount of revshare sites which don't have fuck-all for content, but want $30 to join. (And we wonder why surfers feel burned and will settle for low-quality tube vids.)