It's definitely stiff, although nearly everyone installs coil-overs. No harsher than the twins[/QUOTE]
I owned a Z4M coupe for 3 years so I think I have a pretty good idea of their pro's and con's. While it is certainly more powerful than the twins, it is also about 400 lbs. heavier. The S54 engine was designed about 15 years ago and required very expensive valve adjustments every 40-50k miles. It is also a little bit of a gas-hog with EPA ratings of 15-22. I took it to the track a couple of times and it's a little scary at times. I found the BRZ to be much more predictable and easier to throw into corners without worrying about the ass-end of the car coming around when you least expect it. Contrary to the quote above, I thought the ride was more firm and jittery on the highway than the twins.
On the interior, the gauges seem out of place and cheap compared to the rest of the car. The trunk space is tight and there is no rear seat to fold down to expand its space. Nappa Leather seats are nice but the twins seats were much more supportive.
In summary, I think the Z4M coupe is one of the best looking cars BMW has ever done but I haven't looked back since selling it nearly 3 years ago. While the BRZ doesn't have the panache of the BMW, I think it is more fun to drive on a day-in, day out basis. I had a run of BMW's that lasted nearly 30 years that ended with the Z4M and now that they've gone to heavier and even more powerful cars, they may have a hard time getting me to come back.