I've been driving a 1997 M3 for a little over 3 years now. I bought the car because, well, because I took one for a test drive. As you've probably heard, it's a fantastic car to drive.
Basically everything about the performance is excellent. The motor pulls like a train and the exhaust note is like music to your ears. The M3 also has the most precise steering and handling of any car I've ever driven. The brakes stop the car on a dime.
In the wet, everything changes. The handling is a bit skittish and the car has never felt very comfortable on the highway in heavy rain. I am usually chugging along in the right lane getting blown away by everyone else. It may be that the car always feels planted to the pavement in the dry that I'm uncomfortable with anything less, but I've never like driving the car in the wet.
The interior is comfortable, if a bit tight for anyone over 6 feet tall. The controls are laid out nicely, other than the window controls being down by the gearshift knob. The back seat is also quite usable for 2 passengers.
Reliability has been very good. While there always seems to be something small wrong, the car has always started and run fine. The small problems I've had are things like the steering wheel squeaking a little bit, the clutch pedal squeaking, the gas gauge taking 2 days to go from 1/4 tank to full, the windshield washer fluid not working, etc.
The biggest downside to the car is the hidden costs of ownership. I usually get about 20 miles to the gallon. While this is better than most SUVs, it still isn't very good. The tires wear out very quickly. I replaced all 4 after 2 years and am nearing another replacement. At $1400 a set, it gets expensive very quickly. Also, the brake rotors are a wear-and-tear item on this car. I had the front brake rotors and pads replaced for about $600.
Insurance has been fairly reasonable. I pay about $100 a year more than I paid on the Honda Accord I drove previously.
This is a wonderful car if you have the money and you want a really nice toy that can still haul luggage and passengers without standing out too much.
I plan on getting a new one sometime this fall.