All I can say for me is the following:
Picked car up end of January 2011 a 57 plate 320i. Only problems were a slighlty loose handbrake and seemed to have a lot of steam coming from the exhaust when really cold. After a few days I noticed at idle 650rpm the car shivered or juddered and a couple of times it took a few turns of the engine to start. It went to BMW. No faults that could lead to bad starting could be found, the handbrake was tightened. They did find a fault with a oxygen sensor so it was changed. Seconds after driving the car away from the dealership it blips for the first time, basically feels like when you change gear and don't do a smooth job, your cruising along and then suddenly feel a weird jolt through the car. It did this a couple of times. A few days later it went back for an oil change and they read the codes. Now a fault on the CAT had come up saying it was full. I was told to give it a good run down the motorway which I did. since then it is now blipping all the time and now its developing a jerkyness when under light throttle through the 1500 - 2000rpm range at low speeds. It is going in tomorrow to have the CAT fault cleared as apparently this may be causing problems simply by being there. If anything else comes up then that will be sorted, if not I drive away and if the symptoms persist it will go back!
The 320i problems seem to span from slight jerkyness at slow speeds and missed beats causing this jolt or judder, to full on failure where you get engine warning light and the car souonds like a tractor.
This is my first BMW and I love the car but I'm starting to wish I'd spent a bit more and got the 320d as I have a horrible feeling this will be a 3 year love hate relationship
I intend to talk to my dealer tomorrow about BMW advancement on this problem as I find it hard to accept that an issue which has been arouond for years has not been sorted to stop the repeating fault stories you will read on this thread.