Indeed, if you felt that Civ V was enjoyable but a bit lacking, there's a good chance you'll really enjoy Gods and Kings. The game just feels like it works better now, with the changes in combat, tech tree, diplomacy (to an extent) and overal balance being the main reason for that. I haven't played a very large amount of games yet (I've started a bunch, completed 3), but I'm really liking the religion aspect of the game; the idea of using faith as a new ressource that exists separately but can be interwoven into the managing of other ressources works really well. There are some balance issues brought on by some of the new additions (most people will think of Austria when reading this), but these will likely be fixed in upcomming patches.
I don't think that Gods and Kings will change the mind of someone who really didn't like Civ V, as it basically solidifies and improves (significantly, in my opinion) the base game rather than makes drastic changes that some dissapointed fans would have liked. For anyone who would have liked Civ V to simply better executed, I think this expansion is a really good step in the right direction, and has made the game more enjoyable than ever.