Theoretically a good addition that just doesn't really do enough.
Really my main issue here is the much vaunted changes don't actually do much to the game. First you have religion. It's nice, you have a system where it spreads, it has it's own currency and such, but you find in play it is just a glorified wonder building for the most part. It gives you some stat advantages, a pretty icon, and is promptly forgotten. It definitely is a nice addition, but just not all it is cracked up to be. Espionage is a bit better, but coming in late game kind of diminishes it. You can again quickly forget you even have these spies to work with, save the occasional nuisance message about a tech being stolen or such.
They added some new civ's... yay... not that big of a deal really. You have some melee ships now, which I found rather useless as city of decent strength will laugh at their attacks, and you still can't attack land units (in fact, you have even less ability to do so than you did before the expansion, as at least then you could used ranged attacks on them). In the end, the combat feels the same. I'm sure there is some tweaks in there, but I'm not just really seeing them.
My biggest gripe is they dis-stabilized the game again. Civ 5 started with that issue, but it got mostly fixed, but now we seem to be right back to vanilla instability issues. I lived with it in Civ 5, and I'm sure they will patch it, but with such an underwhelming experience I just can't be bothered this time.
The take away I think is this: Are you still playing Civ 5? If so, this is probably a good purchase right now for you. Unlike me, you'll probably notice these changes the most and it will refresh the experience. Likewise if you really liked Civ 5 and just looking for something to freshen it up, you may also want to look into this. For me though, who was already mostly done with Civ 5, it doesn't freshen it up enough. For people like me, wait till they come out with a steam sale for it, or skip it entirely.