Depends what engine your going for
The 35i can have issues with the Fuel Pump,
The 20i have some people report issues with a whine when cold
Rattles from Roof and in some case leaking roof,
Early cars had leaking rear lights, BMW carried out a "Product Enhancement" to fix this, its not a recall so may not have been done, but if it hasnt BMW should do it FOC
People suffer from "Dancing Windows" where the windows go up and drop back down, mine does it sometimes when putting the roof up, not bothered going to dealers about it.
296 19" wheels crack on the rear
The standard speakers are apparently poor, I have the upgraded business system which IMO is good enough, I have not heard the standard system so cant give my opinions on it
Probs more, but thats all I can think of ATM