As a "heavier" cyclist (250 lbs, 6'4") I've had success with Shimano SPDs, and SPD-Rs. No problems whatsoever. I ride about 2,000 miles in the summer, plus (who knows?) how many miles on the trainer in the winter.

I just got Shimano SPD-SLs and will be trying them soon (I'm having a difficult time getting the cleats mounted on the shoe; I can't get the cleat screws to catch, they seem too short). I really like the platform on the SPD-Rs and the SLs (at least by looks)--nice and wide for lots of stability. Makes me feel very comfortable when standing.

That's my two cents.

EDIT::
Rode with the SPD-SLs tonight for the first time...25 miles. The SLs are SWEET!! Easy in and out (though it took time to get used to the clip in angle). Once I got used to them, they were great. Nice stable platform, and the easiest in/out I've experienced (though I've never tried anything but the various Shimano SPD series). Purposely pushed a lot of hills out of the saddles just to give them a test...felt like climbing stairs.

Had to break down and get new shoes at the LBS (Sidi's). Seems my old shoes (Diadora) were not really compatable as the manufacturer suggested. Loved those old Diadora too.