[mindcrime]

As far as I know Shimano and SRAM are compatible,

My understanding is that they aren't, at least where we are talking about indexed shifting (which is pretty much everything these days). The amount of cable pull per "click" is different between SRAM and Shimano, which means you can't use a Shimano shifter with a SRAM derailleur. Or maybe that's just true in MTB world and not road bike world, not sure.

OTOH, chain, cogs and chainrings are pretty much compatible, at least within the constraints imposed by the changes in chain width to accomodate the newer, denser cassettes. Eg, 10 speed chain or 9 speed chain won't work with an 11-speed drivetrain.