Gustav,
I've had my M5 for nealry 5 months now, and have covered about 5k miles in it. It has done most things, from popping to the shops to teh occasional high speed blast on the motorway. It never fails to impress. The finish is better than on my previous BMW (and E46 330d) and I've not smiled so much for a long time when driving!
I see JJJ's point about the number of owners, but to be honest, a car is only as good as it's condition, service record etc. I'd rather have a car with a number of former keepers that was in mint condition, than a one owner car that was a bit tired. One owner a year is not a problem IMHO.
Running costs are OK. I'm getting 23 mpg as an average, but then much of my time is spent on motorways, so I guess this helps. It did drop to 4mpg at Donington, but we'll not talk about that!

Servicing will not be too bad. I'll provide my own Castrol, as BMW dealers charge extortionate proces for oil. The other parts will be provided and fitted at the dealer to keep FSH.
Overall, my dealer has been great. I've had the VANOS unit u[dated as it was noisy at startup, and the warranty extended until April 2004. I think that this is probably the most important thing, as a new engine and gearbox can cost as much as a MINI Cooper S. Better safe than sorry.
Downsides to ownership are the constant niggle that if I leave it anywhere it might not be there when I get back, hence the tracker. Insurance is just over £1200 with protected NCD and my girlfriend on the policy.
The brakes are the worst feature of the car. How BMW think that they can put a single piston sliding caliper arrangement on what is effectively a supercar strikes me as daft. Still, a nice AP 6-pot kit will soon see to what I believe to be the only real failing.
God I love this car!
Matt