I'm a multiple-time BMW owner and self-proclaimed pseudo-expert, trying to get into the Boxster realm.
The M roadster is an awesome, awesome car, but the key here is the chassis it's based on - the not-so-modern Z3. Even when "M-ized", it's got a bit more flex and isn't quite as sure-footed as a Boxster. It is, however, rear-wheel drive, which you may or may not prefer over the Boxster's mid-engine setup. And a BMW inline 6 (especially an M motor) is a glorious thing to experience.
The M coupe, on the other hand, is substantially more rigid and will give you top-notch performance. But it's not a roadster and I personally think it's butt-ugly.
The Z4 is considered to be on-par with the Boxster with regards to handling, and having driven one on multiple occasions I can say it is an absolute blast of a car to drive. The upcoming M version of that car will probably be disgustingly high-performance.
As far as reliability goes - BMW isn't king of the hill. They aren't bad (their powertrains are particularly strong), but all the electronics tend to go over time. Motors, switches, sensors, etc.
The one thing BMW has done that keeps me coming back to them is balance everyday drivability with performance, safety and incredible value. While many cars are a "9" in some things and a "2" in others, BMWs are typically "8"-"9" in *everything*. You can switch from a subtle, smooth ride to tail-out-happy-fun-times with a flick of a wrist and blip on the throttle.
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