I am not sure what all these Z4 owners are posting here, but the Z4 feels like a toy in comparison to the SLK350. First, this car handles very well at high speed. The Z4 seems like is going to loose control. Second, the hardtop is a huge plus. It accelerates very well, in the mid 5 secs, and the engine sounds like a race car. I have had this car for almost 4 years with only one minor problem that was fixed during the first maintenance. It has been the best car I have ever had.
Acceleration, hardtop, looks
There are a few things to improve. First, the seats are not very comfortable for long rides. Nevertheless, after almost 4 years they look almost the same. Second, the material they use over some of the plastic parts starts to peel after a while. I would re-position the cup holders too.