I've driven an acquaintance's 2011 Mustang & honestly, the ride's pretty smooth & yup, it's equally as fast as a 6-speed regular manual e92 M3 (7-speed DCT would be slightly quicker). It's comfy but nowhere near as comfy as in the M3, the build quality's lacking (compared to the M3), I really can't stand the cheap horrendous looking interior & I'm not feeling the deep low v8 rumble as I much prefer the high-pitch Euro v8 sound (personal preference). Yes, it's pretty much a drivers race in a straight line & on the track (again, w/a 6-speed regular manual as the 7-speed DCT M3 would be a tad quicker). My point is, a potential Mustang buyer wouldn't be cross shopping an M3 & vice-versa. You buy the Mustang for the bang for the buck (nothing wrong w/that) & you buy the M3 for the great luxury & sport combination it provides. If I wanted all out performance (w/no luxury) then I would've gotten a Z06 for roughly the same price (when you factor in option packages for the M3).
As for the maintenance, BMW covers everything (besides tires) for the first 4-yrs/50k miles. Only thing that sucks about European cars is that mods are much more expensive but for the obvious reasons such as most aftermarket parts made in other countries then getting shipped overhere & these manufacturers know if you're willing to pay at least 2x more for a Euro car then you can afford to pay at least 2x more for aftermarket parts while most aftermarket parts for the Mustang are made overhere, etc.
Do I like the 2011 Mustang?!?...Yup, I sure do as I think they're nice cars for what you get (especially against it's true competition, the Camaro & Challenger). However, if I had to do it all over again, there's no way I'd be considering a Mustang as an option for the reasons already stated. I've had my M3 pretty much since it came out 2.5-yrs ago & I love driving it every second I get (it's my weekend toy). It's not always about performance as I love the comfort, luxury & build quality it has. Am I a douche bag, pompous arrogant a$$hole many peeps perceive a BMW/Porsche owner to be?!?...Nope, I just love my M3 & everything overall it has to offer. I didn't buy it to impress others and/or pickup chicks. I bought it b/c (again) I love everything it has to offer. Then again, I love my FJ Cruiser since it looks like a life sized Tonko toy, LOL!
Hope that helps.
- Jimmy