1 year ago, I traded a 2009 M3 DCT for the 2013 ZL1 M6. I am happy over all. But, I miss the quality interior of the M3. I especially miss the DCT (really miss that). I am shocked at the fact that the M3 was actually faster in real world use (with only 414HP vs 580hp --- My fellow ZL1 owners can hate on that or cite ring track times, but until you drive an M3 and Drive it past your personal limits, you will never understand the razor handling and ease of use that is the M genious). The M was more consistent off the lights and handled much, much, much better - especially in tight mountain roads. Where\when the ZL1 starts to feel like a cow in a turn, the M3 will feel like a 2800lb sportscar. However, I am happier with the ZL1 over all. I like the looks it gets - EVERYWHERE, by EVERYONE. I love the exhaust note, I love the torque, even with the unruly wheel spin. I hate the weight.
A big one for me, being a current corvette owner, is ease of modification and availability of parts. The M3 had little in the way of mod options, most were super pricey and gave little to no HP benefit (aside from supercharger kits for $13000 + install) Another problem with modding the M, is your warranty, BMW was super strict on me. GM seems to be more lax on modding and abuse. And the after market for the camaros is huge and growing daily.
However, if you are a corvette guy, the weight will nag you, everytime you start off in first gear, while in traffic. Otherwise, I think you will love a ZL1
My list of problems so far:
Tires lasted only 12000 miles
Double clutch is clunky
Supercharger rattles like a can of marbles.
Rear differential grinds and groans, been in shop 2 times already, has to go back again.
brake caliper paint bubbling off rear caliper.
Brake master broke, needed replacement
brake reservior, cap leaked out and into engine compartment.
Mylink system is glitchy, fan mode gets stuck, bluetooth won't sync or unsyncs weekly.
Wheels are only powdercoated on outside surfaces, WTF GM?!
Gets 11 MPG, maybe 11.9
rear mag ride shocks covered in a film of oil, no idea what is causing it yet.
Brake dust form stock pads is criminal, clean wheels and 2 drives later they will be nasty.
My list of problems with BMW was much, much, longer though
