Depends how short the legs of the front seat passengers are... I am OK at 6'1" in the back of E36/E46 BMWs if the front seat is forward-ish. No hope if all the way back.
I'm 6ft and I've sat in the back of a 330i convertible and thought it was OK (an M3 should have at least the same amount of room, if not more if it's a coupe).
the leg room in the E46 M3 coupe is fine. But if its a major concern it might be worth waiting to see if BMW put the M3 saloon into production. The difference in legroom between the E36 M3 coupe and saloon is noticeable. Chances are the same is expected of the E46.
