Maintenance, etc:
I've ridden and worked on both. Shimano is ultra smooth, when its on, but its fiicky. If your cables gunk up a little bit, the shifting gets all jacked up and is a pain to adjust. I think it has to do with the shorter pull of the cables.
SRAM on the other hand, I've found that its virutally a "set it and forget it" drivetrain. Rarely do I need to adjust it, and to be honest, I dont clean up the cables often. This season has been particularly harsh on drivetrains with all the moisture we've had. I'm very impressed with how my X.O. stuff has worked.
Shifter feel:
Shimano has a really light feel where as SRAM has a positive "click" when you shift. This is personal preference, but I like the SRAM better.
Cassette/shift quality:
I dont think there's any question that the Shimano cassettes are better than the SRAM. The SRAM cassette is very clunky whereas the Shimano is super smooth and quiet.
My personal preference:
I like the feel of the SRAM shifters, but I like the smoothness of the Shimano shifting. I'm done with SRAM cassettes. From now on, I'll be running SRAM shifters and derailleurs with Shimano Cassettes. I don't think the chain makes much difference, so I'll run whatever I have handy.