OK - I have just test driven the Z4M. The Coupe is not yet available so I drove the Roadster. There follows a very brief report:
The car feels very lairy - maybe I have been spoilt with my 997 but there is very little composure and fluidity in the cornering. I had the back drifting out on virtually every low speed roundabout I drove on. Even fast acceleration on the straight made the back feel twitchy and jumpy.
Maybe it's familiarity, but I could not feel confident in this car in the short time that I drove it. Especially with the Sport button selected (which incidentally did not change the "feel" of the car in the same way the Sport button on my 911 does), I simply did not feel there was enough grip at the rear to get me around the corner.
You really have to rev this car to feel the power - at below 4k rpm I could have been driving a 2.5i rather than the M version. As a result, the power does not feel as immediate as it does on the 911 - you have to change down or nothing much will happen.
Steering is light and communicative but not in the same league as the 911 - ie you do not feel every bit of the road underneath you.
The ride is pretty good actually - probably better than the 911 and possibly more comfortable on a longer journey. I had heard negative things about the Z4 suspension so this was a surprise - perhaps it's down to the lack of run flat tyres.
The driving environment is fantastic though - this does feel like a special place to be - grippy seats with loads of lumbar and thigh support (much better than on my 911 although I do only have the standard seats) and a tight cocooned feeling yet not claustrophobic (I'm 6"3). The steering wheel is far too thick however - great if you like that but I prefer the thin multi function wheel on my 997.
Lastly, engine noise was good but more of a whine than the grunt of the 911. At low revs it was nothing spectacular.
Overall, I enjoyed this car, but feel that it does not flatter my driving the way the 911 does. It doesn't feel as fast, yet it feels less controlable. This could be overcome with familiarity perhaps. It requires respect and attention when driving fast - consequently I did not feel nearly as confident driving it quickly - eg a 50mph bend in the 911 would be taken at no more than 40mph in the Z4M.
I will wait to test the Coupe but I don't think it's the change I want right now. Perhaps I will put my name down for the next M3 or just order myself another 911.