In general, how reliable is the BMW 12 cylinder engine?

I ask because every Mercedes Benz has steered me away from their 12 cylinders. I did fall in love the the 04' SL600, I had one for a couple of days for a test drive and it was ungodly fast, but again the reliability factor. MB didn't seem to think the warranty helps other than to defray costs.

The Ferrari comes out to about $2 per mile for maintenance, how does the BMW compare?