The Z4 is fairly reliable. Mine is seven years old and I don't have any major problem with it.
The roof is squeaky when retracted but that's pretty much all convertibles.
The power window regulator may go out (3-series weakness).
The weather seal around the windshield tend to deteriorate. Keep an eye on it and once you see what looks like dry-rotting, it's time to replace. Alas, usually you'd have to replace the windshield too. I didn't mind replacing my windshield along with the weather seal because my windshield was chipped and all crappy looking.
Door handle mechanism may go out after a long while.
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Cogito Ergo Vroom - I think therefore I ride
 
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