Mine isn't a 335i, but I don't think the engine variant affects top integrity. That being said, as Fun2Drive mentioned I am a heavy user of my top. (had it up and down at least 5 times this last week of November).
I have a little teen-einsy drip that happens every once in a while if I hit the car just right at the passenger side a-pillar while washing the car. Other than that, no leaks.
Mine is a July 2009 (pre-LCI) build, took delivery Aug 2009.
I did have a sticky front hinge mechanism that was replaced under warranty and with an SIB in March 2011. Also, had the headliner tensioning cords replaced under warranty in August 2011. The frayed cords have been addressed by 2010 and newer cars getting the more robust cords (also what they used in my replacement issue).
I've never heard of the top sticking due to a failing battery - that's just crazy. It is true that actuating the top repetitively, without the engine running is a huge draw on the battery, but as long as your battery is in good health, I wouldn't sweat it. Unfortunately, or fortunately, depending upon perspective, I live fairly close to my workplace so my car suffers from short hop driving. That has been its biggest issue. I just had my battery replaced under warranty and have acquired a battery tender to keep it properly charged.
I am a firm believer in keeping the rubber seals between the panels well lubricated and moisturized and as a result my top is creak free, and the car is very quiet (no wind noise) at highway speeds) - in fact it's quieter than the Honda Civic coupe I previously owned.
If you do decide to go this route - and I personally think it's a good route, make sure your local preferred dealership have service people that have gone to the cabrio maintenance school. The shop foreman at my dealership (large - high volume in Houston) explained to me that there is a special class techs have to certify in to work on the retractable top.
Good luck with your decision. I love my cabrio and the pleasure it gives far outweighs the effort of the extra maintenance required.
If your sport doesn't put blood, dirt or grease under your fingernails,
then it's just a game.
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