I think it varies a lot within the M3. A very clean E92 M3 in a nice color with clean, shiny ZCP wheels (or aftermarket) can be quite a head turner whereas a dirty E90 in white or silver will blend in with background traffic.
I am not able to keep my M3 garaged (city living) and it is black so it is always dirty. I find that it is actually pretty inconspicuous. Which is fine by me because part of what I love about the M3 is that it can be a bit of a sleeper. Fast, nice car that Joe-Cororolla sees as just another small BMW.
That said, I remember when I went to pick the M3 up, my wife drove it back home and I followed her in my 997 C2S and I saw more people checking the M3 out than the 911 during that ~150 mile drive. Then again it could be that she was an attractive blonde in a new M3 and I was just some dude. But I remember being surprised by that.