I think most of the M3 and M roadster guys are enthusiasts. There are a lot of BMW auto-xers. Now on the other side, there are BMW owners where I work that couldn't care less about performance.
Here's how I have come to see it. This is just a generalization based one personal observation.
M3's, M coupe, M roadsters: owners like to drive with authority
M5: owners like power and luxury. Some off these guys considered the M3 (2000+)
540: owners want nice luxury with good power and something bigger than an M3.
528: Just wanted the luxury or just wanted to be able to say "I have a 5 series."
330ci: didn't know there was an M3 for practically the same price.
330xi: Simply prefered it over the S4
330i: felt it was much better than the 325i, and likes the overall feel.
325i and lower: just had to have a BMW reguardless of how low it's power is, or how much luxury it lacks. These guys really like their cars and most wouldn't buy anything else for the price. Why? Other than the label, I just don't know.
Z3: owners like a fun car and wind in there hair.