When I was at the dealership for my 1200 mile service, there were 2 Z4s there available for sale. I took a look at it and here are some impressions.
1. The hood looks very very plain. No personality. Compared to the M5, M3, X5, etc. It's just 1 piece of smooth metal.
2. What the heck are those side directional lights? They are BMW roundal protuding with yellow light around the side. Very very ugly.
3. Noticed the line across the front of the car actually continues to the side pannels and downwards toward the front edge of the doors. Hm ... I wonder if the bottom portion of the car rotates like a transformer?
4. The butt look very very good, compared to a Z3. IMHO. Much more defined and has a more agressive look.
5. The sales guy told me that the Z4 has the same wheel base as the Z8. And ... it looks like that a 8 cylinder can fit under the hood easily! Could this mean that BMW has future M versions in mind? I think so! (let us all hope!).
6. The head lights ... more boring than a MB. I almost fell asleep looking at them.
Over all, I think that the Z4 is a replacement for the Z3. Not a good one but not bad either. I just wished that it had more of that special BMW touch, which makes all BMWs special?
My 2 cents.