OK, here is my opinion on this
I have been a Huge Map/Marathon CIV IV player since I got Vanilla for Christmas 2006 (going on 2 years). Have both expansion packs. Predominantly Prince in vanilla/warlords, Monarch in BTS (but migrating to emperor)
Reason for going to marathon? The games were way too fast, I was going off to war only the have an army I just created go out of date real fast. I sill like marathon for these reasons and probably will no change although I do understand it is not as balanced as the other speeds (I have downloaded a few games from the forums and just couldn't play at standard speed).
I played huge maps, well because the world is just so damn big!! My games ended either with a space victory or UN election (because I dominated the game ansd elected myself victory) or the occasional cultural victory. The past month I have started playing standard maps, mostly due to the game slowing down alot because of the late-game BTS stuff. There is quite alot of good improvements in the game (warring, more strategic space race, corps) that I do not want to miss.
One thing I found is the games play alot differently, and I like it. Dealing with AIs are different as we all have closer bounderies. It's also easier to know a few AIs out early and deal with a smaller "diplomatic" world. And domination/conquest are real possibilities now.
Also on huge maps, the economy can be real horrible. You cannot expand far enough to culturally touch an AI without tanking the economy (too many games I built the forbidden palace before culturally touching an AIs borders). Now it's not the case, 6courthouses for the FP, 6 universities for oxford etc... Even cultural requires only 9 temples to get 3 catherdrals in each legendary city.
I may consider moving to epic speed but right now enjoying standard sized maps with marathon.
And most importantly, the game moves along at a good speed even at the end on my 5 yeasr old Dell.