I'm about 165 pounds, ride XC-style stuff, and would prefer to run about 35-40 psi front, 40-45psi rear. However, when I get into rocks, this earns me pinch flats unless I have, say, 2.3" tires. And 2.3" tires have some clearance issues on the rear of my particular setup, not to mention that it's hard to find light XC tires of that size. So I end up more like 45-50 front, 50-55 rear if I use 2.1" tires.