My humble two-cents worth.
I've had all three guns side-by-side and my observation is this: If you like the ergos' of the Sigma, you'll feel right at home with the SD. Grip angle and feel is very similar. However, when you look at the slides of all three guns, the slide on the SD is very similar to that of the M&P. To me, the SD looks like the mutant offspring of some kind of unholy union of Sigma and M&P. The good news is, the trigger is much improved over that of the Sigma and that ridiculous Sigma-only rail is gone and new standard accessory rail in place.
Now, keep in mind, trigger is not as good as the M&P. Like the rest of the gun, lands somewhere between Sigma and M&P. Price point also lands somewhere between but, is only $100 or so from M&P pricing after the $50 rebate currently offered.
If you have and like a Sigma (you should have you head examined), you'll really like the SD line.