I compared my M2s (yes M2) with my friends Sennheiser IE8 (definitely genuine; he bought them from Sennheiser directly) a few days ago. I found them to sound rather bland, unlively and boring. Much more balanced than M2, yes, but I want to enjoy my music, not analyse it. I didn't believe my ears as I expected them to be A LOT better than M2. I listened them on my M6 TS with flat EQ, everything turned off except Spatializer (PCE level3), but I compared them without Spatializer as well. Maybe I prefered M2 because of lower impedance, but even when I turned on the volume on IE8, they didn't impress me much. Bass was more interesting on M2, although IE8's bass knob was turned to max.
I guess that this is due to the silicone tips my friend had on his IE8 not perfectly fitting my ears (it were those tiny bi-flanges). I'll try them again with other tips, maybe they just require a good seal to show their strengths.
So yeah, I guess the M3s must be awesome, considering I'm already really amazed by the M2s.