I've just got a decently sized Eldar army, about 16,000 points worth. Their the only army I have but between all the possible list combinations I haven't had to play the same list twice except for a few times. It is getting a bit more common now days though, as I tend to get a couple games in a week.
I don't mind Meching up, but I have had quite a bit of luck with my Wraith units and Warp Spiders. My methods against/with any given army/list may change as that is just what is called for. Any week I can expect to play: Eldar, Khorne CSM, Nurgle CSM, Space Wolves, Catachan, Tyranids, Salamanders, Ultramarines, Sisters, Necrons and rarely Dark Eldar. We have a nice variety I think, so my tactics cannot afford to be set in stone.