I have zero experience with them but I've read that the 3-series at least is relatively cheap to maintain and repair. Hard to believe it could be any worse than Volvos or VWs.
The "quite a bit of preventive maintenance" is the same that I would have done on any car that age. The cooling system is a known issue. It works great, but the plastic expansion tank is a weak spot and will eventually crack and dump the coolant. If unsure of the age, at least replace that. It isn't expensive.
I really do like the e46 BMW. The design still looks fresh today. I was talked into buying it for my daughter as a first car by a friend who has 3 of them in his fleet, and knew what to look for in one. I was reluctant, but a year and a half later, it's been a great car. It looks almost brand new inside and out