I have driven one, i was considering a change last year i was going to move from the E46 M3 to one, mainly because of the deals that were going on the M135, and figured (using man maths) that it would be cheaper to PCP an M135 for 3 years, that it was to buy the M3 have it depreciate for 3 years, then replace all the bits and bobs that i had to.
Anyway, fantastic engine, new...... but that was about it to be honest.
Felt a little loose for my liking, and not really a special car...... a decent drive in the M3 changed my mind VERY quickly.
New car i would suggest at the same price range is the GT86.... which is a VERY VERY good car!....... or you can stick with what you got! but i know what its likes to want a change.. i have have the Z4MC for about 2 days now, after having the M3 for 3 and a bit years... i only changed because i wanted a change.