I wanted to LOVE this place but my reaction every time I go is mostly just 'meh'.
If you've got a lot of time to spare for the day, I imagine it could be quite a cool experience to just shuffle through the shelves and look for something of interest. I personally find it a little difficult to find anything with the way they are organized but that may be partially due to my own lack of patience. Even though I am a book lover, I actually prefer their non-book sections. They have a large selection of movies and tv shows there that I've found things to buy in. They've got some other cool memorabilia there like older magazines, posters, comic books, etc.
The thing I really need to emphasize though is doooon't bother 'trading in' your books to them. Seriously, you'd probably have better luck with selling them online. I took a ton of books into there and hardly got anything for them, and that seems to be the general consensus with others who have traded in items. Even if they are in good condition, you won't get what they are worth back. You're lucky if you get $5 in return.