just to add to what a2 and others have mentioned:
SRAM does have a line of shifters that are non-esp meaning they are shimano compatible. the xray, rocket, and attack come to mind these use 2:1 actuation ratio like shimano systems.
SRAM's higher end lineups use their 1:1 esp system. from my experience, using a esp shifter with shimano rear mech will result in 4 clicks/8 cogs.
one thing to note about SRAM is that they have gotten considerably more durable in their components with the use of forged alloys and less composite material in certain stress areas.