Well, it really depends on who's directing, producing, acting, etc. A movie adaptation could end up being really good or really horrible. I mean, personnally, I think X-Men and Spiderman turned out well (although I haven't seen Spiderman 2 yet) but there are also some really bad comic-based movies (apparently Daredevil and Scooby-Doo weren't very good, although I haven't seen either).
Also, I liked the Lord of the Rings and Harry Potter movies, and yeah, I did read them way back when, "before it was cool". In those books, we didn't even know what the characters looked like, let alone how they talked or moved. And yet I think the movies ended up being really good, especially Return of the King, which was a great way to finish a trilogy. Haven't seen Azkaban yet tho.
Anyways, like I said, it really depends on who works on it. Any concept has potential, it's just how well it's executed that matters.