What I have and how I rate it, in order of value:
Mongols - Free, and a nice Civ to have in the game. Their UUs are fun to play with, though arguably a bit overpowered. Obviously worth it, best value since, well, it's free.
Denmark + Explorer's Pack - A very interesting civ. Their UA (especially the pillaging without spending movement points part) makes fighting war a lot more fun and profitable. Their UUs are actually pretty fun to play with. And the Explorer's Pack is a fantastic DLC as well: Sandstorm is my favorite random map script, and the premade maps are actually pretty good (I really liked Bering Strait). Absolutely worth it, I got more hours from the Explorer's Pack alone than I've gotten from some U$20,00+ games.
Spain + Inca - Two Civilizations with very unique abilities, units and improvements. The scenario is fun, too. For being a bundle of two civs, this has pretty good value.
Korea + Wonders - I don't like this one as much as most people around here, but I still like it enough. I like having Korea in my games, but I don't like playing with them that much - Research focus is already a bit overpowered, and I'd rather not make the game even easier by playing with a Civ that facilitates it that much (reason I probably won't purchase the Babylon DLC). The Wonders pack is nice, but I'll be honest that paying U$5,00 for just three buildings is not really good value, even less if you aren't going to play the scenario (it's a nice scenario, though). The bundle has just the right price, not exceptional value or anything, but not overpriced.
I still don't have Polynesia, but I want it. Seems like a very different and interesting civilization. I have a thing for Unique Improvements, and the Moai Stones seems fun. Too bad the AI is pretty bad at Naval management, though, making pretty much any Archipelago match a walk in the park.