I've only got 1500 miles on it and most of that has been winter miles on colder tires. But...it's pretty awesome. It is just as quick and definitely handles better. It's more comfy inside as the seats are 14-way adjustable (I had cloth Recaros in the Mustang that were decent). The clutch and shifter are much lighter and I could easily DD the M2. I enjoy all the bells and whistles that I didn't seem to care about. I get more looks and thumbs-up with the M2, but overall it blends in to most of the population. I also like that the M2 needs nothing: the wheel gap is small, the wheels look great and the offset is right, and the brakes and suspension are fantastic.
I have a nice loop around Round Valley close to the house that I've hammered in the Mustang, Focus ST, a friend's C6, and now the M2. The M2 was VERY buttoned down at ludicrous speeds at 40-50 degrees out. No fanfare; no muscling the car. Just sticks and goes.
The Mustang was a great, cheap, fun car. The only thing I miss is the sound of the V8. The BMW is nice but it's no Coyote. I ordered a catted downpipe and will put that on next month sometime. I think that will help some.
Is the BMW worth nearly twice the price of the Mustang? I dunno. That depends on the person. I am sure if Ford could actually assemble a S550 Mustang correctly and I threw $5000 at the suspension, exhaust, and wheels and tires, I would have been very happy and saved some cash. So I'm not sure I'm a BMW fanboy quite yet, but I don't regret writing the larger car payment check each month.