I've been thru a long journey with my 50mm 1.4.
Initially when i bought this lens, I had only my 1D2n around. The lens paired perfectly and was the first copy I try at the camera store. I bought it instantly.
Later on I picked up a 5d but this lens was ever so slightly off. I didn't want to calibrate the lens to the 5d because I was afraid it would screw up with the 1D2n.
A few months back I bought a 5d2 and this lens was way out with it. I decided to sent both the 5d2 and the sigma 50mm to their service centre and watched the technician calibrate it for me. It was much much better after calibration, however I could never get that 'perfect focus' look that I used to get with the 1D. Surprisingly...now my 5d has that 'perfect focus' instead of the 5d2
I tried playing around with MA on the 5d2 and I was chasing the scales from -ve to +ve MA. Like I said...the focus was always slightly off and I could never get the focus to nail perfectly. Now I've pretty much settled on two MA values that seem to have a higher rate of 'perfect focus' look. -3 and +10
It kinda scares me that there's no linear effect when adjusting MA but this is only with sigma lenses I guess. I had to MA my canon 70-200 with the 5d2 slightly and that gave me consistently perfect results.
At the moment I'm kinda fondling with the idea of swapping out my walkabout prime with a 35L instead...not interested in the sigma 35 as I'm tired of focus issues.