It's a fine game, but gets boring really quickly if un-modded. I don't know about Civ5, but the 4th is surprisingly bland gameplay wise (bts is just a gimmicky add-on). The combat is mindboggling weak, not even to par with ancient Call to Power 2, which I find generally better from Civ4.
Yeah, you have upgraded graphics, leaders that mostly are fine, but the lack of content is - again - surprising, especially in civics and combat aspects. Espionage is a chore, the growth mechanic is irritiating, religions/corporations don't really matter much, technology is boring.
Of course it's fun to play up to the certain point, it's not a bad game, but personally I would rather launch Call to Power 2 once more, which decades ago handled combat better and Civ up to this point can't simply implement obvious improvements offered years ago by CtP2.
Maybe I will try some mods or Colonization, but I'm fed up for the moment.