Guess it's time to start playing this now

I picked up this game when it first came out, played it for a week, didn't like it and then decided to go back to Civ IV.

Now I'm deciding to give it a shot again. I know there was a new expansion pack for the game. Is it worth picking up? Or should I stick with Vanilla?

I didn't get the game when it first came out, but I liked Civ5 vanilla and then got G&K, and I really like it.

I guess it really just depends on your own preference whether you like it now or not, but IMO G&K has really improved the base game and is worth checking out. You could try a demo first if you like.