I haven't shot an M1A/M14, but I find the M1 rather pleasant to shoot. It gives a good shove, but not a hard kick. I could shoot mine all day with no form of padding, just a teeshirt between me and the steel buttplate.
I used to be very recoil sensitive... when I had lost weight due to a stomach problem, I used to have a .303British that would hurt after about a dozen rounds. Now that I'm back up to weight, even a little heavier, recoil doesn't bother me as much.
Again, the M1 is a pleasure to shoot, just gives you a good shove.
On that note, has anyone noticed in Saving Private Ryan, the M1's they have don't kick? I'd be willing to bet I'm heavier and sturdier than that Matt Damon prick, and the M1 rocks me back, so it would probably knock him on his ass! hehehe...