well to me the S4's were to bright, sibilant, and metallic sounding. the mid-range, especially the vocals on the S4's sounded very distant. i liked their bass and i liked their fit, but that was about it. the M2 are comfortable as well after a couple days of getting used to them.
i much prefer the M2's and it seems the M3's are even better.
I really disagree about the M2s being S4 killers, since I reviewed them for Brainwavz also. I like the M2s a lot, but the S4s sound brighter to me and they are still the most comfortable IEMs, next to the PL50s, that I've ever listened to. IMO...
The M3s have opened up with about 30 hours of general music break-in. They still seem really centered in the lower mids and bass range, but they are getting sweeter in the upper mids and trebles. They really do sound like a good set of full-size, open cans. This is with the medium silicone tips. I'm shifting to the included foam tips later today, but for the moment, I'm just enjoying these excellent sounding phones... and NFL Red Zone.