Shimano and Sram shifters and mechs have different throw rates I think Sram is 1:1 and Shimano is 1:1.2 doesn't sound too far out but getting them to work together is ni on impossible across the range, so you'll need to stick with the same manufacture or both shifters and mechs. Sram and Shimano cassettes though are fully interchangable and can be used with either gear system (just not with that silly overprices Italian stuff)