"If it's a daily driver, get the 550i..
Why the M5? You already have a 997 GT3 (imo one of the greatest track cars ever made) for track days."
As a matter of fact, I'm retiring my M5 this summer and going a step below a 550i. I'll be dropping four whole cylinders down to a F10 535i, which I have on order for ED in July. I think it'll be a great daily driver.
"Well, the difference is that cars like the M3, Audi R8, 911 GT3, Cayman S etc have an actual enthusiast application; the M5 doesn't, really. But it's not exactly designed to be a daily driver, either. It's kind of like an SL600 Benz or an X6M in that regard (though the M5 is probably a lot more fun than either)."
You're right in this regard. BMW wanted an all-purpose luxury sport sedan with the M5, and the result is something in between a daily driver and a track car, while not really entirely satisfying either of these needs. Truthfully, there is no "one" car that can do it all.
"But yes, I do understand having fun on the highway, of course. The E60 M6 'vert is a dream car of mine. From the perspective of someone who can afford it and wants a fun commuter car, the M5 makes sense (though the E60 must get awful gas mileage). I personally would just get the 550i and put the 40k or so I saved toward a more pure "fun" car."
Right on, this is the plan for now. For the time being, I'll enjoy the cost savings and better gas mileage with the 535 (try 12 mpg and 180 mi/tank with E60 M5!), and then reassess when the new M5 comes out.