I've bought every iteration of the Civ series, too, since Civ I, including both versions of Colonization. Never got Alpha Centauri, though I'd like to, eventually.
Civ VI is not an instant buy for me even under $20. The 60% range for review rating makes me so little skittish. Has the AI gotten any better? I'd heard it was awful, even for a Civ game.