I had a BMW Z4 for 4 years as my company car, returned it 20 months ago.
Loads of niggling rattles from the roof and the dashboard
Roof goes up & down easily and is quite fast in transit (around 20secs I think)
Car handles well and performs OK (mine was the 2.2 L straight 6)
Uses run flat tyres that work OK but are expensive to replace. No spare wheel.
Had problems with the steering rack which uses an electric drive coupled to a sensor on the steering column - it went wrong and developed an interesting intermittent fault that locked the steering at speed! Covered by warranty after lots of revisits and words with BMW.
Clutch & flywheel needed replacing after only 60K miles - very expensive just outside warranty.
Boot is a usable size and the car is quite roomy for a sports car
I liked the lines of it
Spent a fair bit of time at the dealers for irritating minor problems
Wouldn't have another - of the choices I'd try the SLK.
Current car is an M.. 2.0L with the metal power roof. Not in the same bracket as the Z4 but good cheap fun and 100% reliable with no defects since new 20 months ago. I like it for what it is and enjoy driving it. It's mine 'cos I could afford to buy it cash at the time.
Everyone to their own
Edited to correct brain fail typo