I love my Dinan Tuned twin turbo convertible! I am the 2nd owner and bought a BMW that was maintained very well under extended warranty by the first owner.
If you research the N54 engine ('07-'10 335i models) - you'd learn that certain expensive components on most of these BMW's fail by 80k miles, so if you buy a 335i that hasn't had these faulty parts replaced, you'll spend thousands of $$ to replace each item. That includes a fuel pump, fuel injectors, water pump and twin turbos. I flew in one way to LA to pick up my BMW after evaluating Carfax report and service records that the previous owner provided and verifying that BMW dealership replaced all important parts including HPFP, all fuel injectors, both turbos, water pump, thermostat, oil filter housing gasket, oil pan gasket, intake manifold gasket, oil cooler gasket, spark plugs, front control arm bushings, front tie rod assembly and other misc. items under the extended warranty (~$18,000) and there haven't been any problems for the last 25K miles.
As other owners of BMW's say - either buy a well maintained 335i that had the issues above addressed (with verifiable service records) or get an extended warranty - otherwise you will spend $5-7K a year to maintain this beautiful and fun ultimate driving machine. I highly recommend you request a Carfax report and verify service records before buying your BMW, and if the seller cannot afford to spend $30 on Carfax report and has no maintenance records - you are probably buying a money pit. Don't buy salvage BMW's either - most have odometer rolled back and are in poor mechanical shape. Good luck!
Check for recalls, I had only 1. This thing is a blast to drive, it's just as fast as an M3, and has a much smoother ride. Watch the speedo, it will easily get away from your attention span. On a recent road trip, my wife was crushing at 100mph, she was very surprised.
Between the wife and I, we have owned an E30, E36, E46 and this E93 BMW. As with the others, when it works correctly, it is fantastic. When it doesn't, it breaks your heart and wallet. First off, MPG is horrid. Best we've had is 24mpg. Coils go bad randomly, I've replaced 3 in a year and a half. It's a lengthy explanation, but due to the direct injection, you HAVE to get the intake valves cleaned about every 30-40k miles, and it's about $1000 each time. But the worst is the fact that it eats rear tires like nothing I've ever seen. Like they are made of butter. 5000-6000 miles, max, and it doesn't matter what brand or treadwear rating they are. Summer tires, All-seasons, doesn't matter. Nothing lasts more than about 6000 miles. I've owned this car 18 months, and I just vaporized the FOURTH SET of rear tires. It sees 90% highway miles. Unless you drive sparingly, or simply have unlimited funds to feed this thing, I cannot under any circumstances recommend this car to anyone.
I've driven this car for 2 years and have nothing but accolades for it. the 335i is a fantastic looking car with compliments galore. Having owned a Honda s2000 sports car, I know the difference between a blue blood and a true blood. What the 335i offers is power with flat-out speed and agility to make the quick dodge or turn if you need it. The comfort for two is plush with every convenience. This is a car you drive cross-country to an art show in SF and feel you are a part of the rich and famous. Of course, with a new $ tag of close to $70k with all the do- dads, you are. However, it is not a ultra nimble S2000, so sticking to a super tight turn with the peddle to the metal is not advised.
Great fun car to drive, looks great, but everytime i go to the dealership, they find something that is not under the certified pre owned warranty, and had the car 2 1/2 yrs, replaced run flats 3 times, very expensive tires, 0 miles warranty, finally last time i just went and bought 2 michellin tires for the back which were same as 1 run flat, and had 45k warranty on the full replacement, not pro rated, full free replacement, only thing i am affraid off is that i have no room for a spare in the trunk, so if i get a flat i have to get the car towed, guess i will just buy a tire inflator pump for the road, and get it towed at Bmw roadside expense if i get a flat.