The car has some sort of design flaw that may be related to the design of the OEM pads and calipers. It doesn't hurt anything, but from time to time, you'll be rolling through a parking lot with the window down and hear squeak squeak squeak... like a wear sensor. The pads will be just fine and the rotors will be straight with no runout. The noise will continue off and on -- 95% of the time you'll never notice it. The dealer replaced all my pads and rotors. Made me feel better, but it still goes squeak squeak squeak on occasion, usually right after you have released the brakes and are rolling free, like in parking lot.
I have replaced one of the runflats -- this car has the sport package, so it has the unproblematic Bridgestones -- the non-sport pack car owners had nothing but horror stories about the Bridgestone models on that car. Runflats are extremely expensive, but that seems to be the cost of doing business these days on a BMW. The tires themselves are excellent and are good peace of mind.
I have had some disturbing trim issues that are very un-BMW. My shift knob (6MT) was losing its aluminum finish with only 26k on the car. Replaced under warranty. The rubber gasket between the lower rear window and the trunk split in the center and was pulling apart, leaving a gap in the middle where water could leak in the trunk. Replaced under warranty. The driver's plastic grip recessed into the door was peeling and looked awful. Replaced under warranty.
I have not had any problems with the motor, transmission, clutch, electrical/electronic, or body. It has been exceedingly reliable.
I traded up from a 1998 Volvo S70t5 (not a weak car at all - see my review on this website) that I had had for 4 years or so. I almost always buy used. This car was a CPO that I bought from a certified BMW dealer in January 2009 for $29,900 with the $2000 maintenance tacked on, and all tax tag title, etc. The car itself was $26,000. I usually don't buy from dealers, but they got the car cheap enough to get me a good deal (it was owned by BMW corporate as an executive car as far as I can tell), and CPO was tempting less for the supposed pedigree as for the warranty. I sprung for the $2000 maintenance extension, so when the car's included maintenance expired in 4/2009, I got another two years. I'm not sure I'm getting my money's worth, but all the brakes could have cost me $1000, so perhaps I'm doing OK so far. So, value wise, not bad (yet).
This car's torque is amazing -- it has fantastic grunt from idle, more or less. The clutch is not the greatest -- it's designed for very smooth luxury action, apparently, which means if you like to move fast, the clutch will make things very abrupt. Some owners remove the useless clutch delay valve (CDV) to fix this problem. Currently, my complaint is that I have to slip the clutch too much to get a smooth engagement. In traffic, that's annoying. However, I know that probably 90% of these cars are sold with automatics, so this might be a "who cares?" situation.
Fuel economy is OK in the city -- if you're really smart about it, you can get 20 MPG in moderate L.A. traffic. Highway cruising at 65 MPH, 6th gear, cruise on, you average 30-32 MPG, seriously. If you put your foot in it a lot, plan on 16 MPG in the city.
The engine sound is fantastic, but still luxurious. The car is clearly a BMW -- not tomb-like like a Lexus, but quiet and composed, nonetheless. Sport seats are excellent, and support is superb. Pedal placement is good and the whole driving setup works great for a 6'4" 200 lb. guy. A/C and climate controls are excellent and work as advertised. The HK Logic 7 audio system is, for an audiophile, a solid B+ system. If you want better, you'd better step way up to a car that has Mark Levinson or McIntosh or something like that. I'm quite pleased with it. Moonroof is large and quiet when open or closed. Adaptive xenon headlamps are really fantastic -- they follow the road, but not in a distracting manner. It feels very natural, and yet you definitely miss it when you're driving another car without them.
I am concerned that the interior is starting to pick up some rattles. it's a stiff riding car with the sport package, and the aluminum trim on the dash and console are occasionally rattling over bumps in way that will be difficult to fix (i.e., remove all trim, clean, reglue?).
Cosmetically, these cars seem to do well. Wheels get dusty fast with OEM pads and will be VERY difficult to clean if you leave them dirty for a couple of weeks or longer. Wheels get cleaned weekly for me, and I only drive 150 miles a week! I'm not hard on brakes -- on other cars, I have usually gotten 45-60k out of a set of pads, front and rear.
Paint is good. Panel fit and finish seems fine (mine has never been in any accidents). I have noticed that the plastic exterior pull handles for each door are tight and sticky -- you have to use each door relatively often or you run the risk of having the handle stick open, which means the door won't latch. You have to push the handle back in. Mine seem to be getting broken in. The doors feel mostly BMW-like (thunk thunk), although some seem to have slightly better adjustment with the latch than others. As for other exterior body concerns, the car does ride low, and believe me, that chin spoiler will make you regret ever hitting a curb with it. It looks awfully expensive.
Leather interior is holding up very well with occasional moisturizing treatments. The light grey headliner will pick up smudges from anything if you have that color -- NEVER touch the headliner! You'll have greasy spots all over it.
I am anticipating certain expenses soon -- the AGM battery that comes with the car is extremely expensive, and someone with a BMW scantool needs to tell the car the battery has been replaced for the alternator to adapt to the new battery. However, so far so good with the current battery. In sum, set aside some serious cash for the little things that come up. There's no reason to think the car won't be reliable, though -- motor and trans have an excellent reputation, and the electrical systems seem to have all the kinks worked out.
Overall, it's a great car. I bought it because I don't think BMW can build a reliable and long-lasting twin-turbo car (Volvo can, if that's your interest), so between the 330 and 335, this was a no-brainer to me. BMW has been building normally aspirated inline 6 engines for nearly a hundred years. I have heard isolated reports of engine failure in these cars, but no more than other cars -- possibly the occasional defect, or possibly neglect/abuse. I think if the motor is going to fail, it will do so before 60k miles. I think this one will go the distance, and I intend to keep this car at least another 4 years.