I (my wife) have a 06 330i. Very reliable thus far (~85k miles). Its out of warranty.
The most common complaint on all these bimmers is the power window motors blowing quite frequently. Once a year almost (1/4 doors will blow every year). Apart from that, its not bad at all. All maintenance was free uptil now including brakes etc (not tires though), so cant comment on that yet. I have a feeling the wifey will not be pleased with the service bills (as Walmart cant reset the computers after an oil change)
If it dont have a warranty or maintenance (ask it for free), then dont bother with one, thats my view.