I love Civ 4 BTS it is very good and runs very nicely even on my ageing laptop. But to be fair I probably wouldn't have bougth the expansion if it wasn't for the unbeleivably good Fall From Heaven 2 mod. With that mod installed the game is the best I've played in years.
My experience as well - I liked the Rhye's and Fall of Civilizations mod, but most of all enjoyed the Final Frontier mod.
The combat system of Civ made very little sense, I agree, but most of all I missed the unit customization. I really liked building my own units in Alpha Centauri - well armored but poorly armed garrison units, extremely expensive shock rovers, eventually getting clean reactors...
My main problems with Civ4 were 1) its "this is how history goes" approach to storytelling - "Pity, you want to make a civilization that respects both science AND religion? ha! The Scientific Revolution always invalidates the science bonus of religious buildings! Because that's what the Scientific Revolution did, get it? hahahah." And 2), I don't know if other people had this problem, but the game took longer and longer to make a turn the farther it went on, and all my units started to move slower and slower and slower, regardless of how far I turned down the graphics. It was obnoxious.
By all accounts, Elemental is the game for me, and I put my money where my heart is. I guess I'll get to see first hand what an EPIC beta is like.