Originally Posted by paisan

SRAM and Shimano both use the same spacing between cogs on the cassettes so they are interchangeble. Front derailler compatibility is no issue also, as many people swap their SRAM FD for Shimano because the shimano's are less flexy and shift under load better than the SRAM FD's do. The problem is that SRAM uses a different amount of cable pull for the RD so if you have SRAM shifters you have to use SRAM RD's.

For ultra low gearing you can also throw on an x9 rear and use 36t MTB cassette's. I set up my Roubaix like that for a trip to Skyline Drive last fall. The reason I chose the 12-36t over the Apex 11-32 is I only lost the 11 at the high end but gained the 36 at the low end. I didn't lose any other gears in the middle, so the already big gaps between gears didn't get any bigger and I had an emergency bailout gear in the 36t, which turned out to be too low for skyline and never got used but it was nice knowing I had it if I needed it.