When I hooked up my M4 I put it into a SATA II port by mistake (I was trying to avoid taking out my GPU and missed the SATA IIIs behind it), I realised my mistake after and switched it to a SATA III. I can honestly say I can't tell any difference between the two.
It does benchmark faster, but in real-world application it makes no difference.
I couldn't notice a difference either, but its nice to get the full performance of the drive!