I try really hard to check out a kit and really search through the kit and contents to make sure it's something I'll like... I've learned a few things to look for when I shop, because in the past, I've been drawn in by a pretty preview and pretty CT layouts, and somehow ended up getting the kit and not liking it.
BEFORE I buy, I do the following:
--I look at the papers. If there is a separate preview for them, I spend extra time looking at it, and consider how many of the papers I will actually use. I'm a very paper-centric scrapper, so paper is the most important thing for me to consider in buying a kit.
--I look at my must-have items to see if they're in the kit, and if so, if they're what I like. If not, I consider the color scheme and think about whether or not anything else in my stash could supplement the kit where it's lacking "must-haves."
--I finally look at the CT layouts in the store. Are all of the CT members sticking to the SAME paper again and again, or the same handful of elements? If so, I really question how much of the rest of the kit is usable; to me, a good kit will have layouts that, considered together, use everything from the kit (not necessarily in one layout-- but like, if there are 5 CT layouts, I'd like to see most of the kit represented somewhere in the sum of all of those layouts).
--I carefully read to see how much of the kit is recolors vs. original elements, and look through the preview and description to see that I'm getting what I think I'm getting.
Since I started doing those things, I haven't purchased a kit that I've been disappointed in. (FWP? Yeah, I've had a few disappointments, but if I got something free and don't like it, I trash it).
When I have purchased kits that I don't like, I do my best to create at least one great layout before trashing it. I figure that, to do a page paper-style, it would likely cost what a kit would, so if I've used the kit once, I'm not losing money.
My goal is always to get at least 3-4 layouts out of every kit I buy, though, so I'm generally disappointed if I pay for a kit and can only get one layout out of it.