A mate of mine has just picked a Z4m convertible last weekend, fully loaded with lots of toys inc DTV . I had the pleasure of driving it last night. I have mixed views on the car. Driving around town the ride is very firm, it`s bordering on uncomfortable when in the passenger seat, but more bearable in the driving position. Motorway driving, it covers it`s speed a little to well & is a little louder than the zed, but it was a convertible. Open the taps & put it through the twisties & it`s an animal, on a tight B road It`s far quicker than my Zed, very easy to drive quickly, it does try to understeer when entering a corner quickly but a quick stab of the brakes before you turn in solves this, get the pedal down & the back just squats down & slings you out of the corner, the engine note and power delivery from 5k to the limiter is addictive. The brakes are good, taken from the e46 CSL I feel they are a little over assisted, as are the brakes in the 370z (my personal opinion) The steering is very direct although spoilt by a fat steering wheel that saps up some of the feel.
Although the Z4m is significantly quicker on the road than the 350z & probably the 370z. I wouldn`t sell up my Zed for one, The Zed makes you work for the rewards whilst the BMW just gives them to you. But if I was was looking at buying either a 370z or Z4m I would probably go German