I know from previous experience that automotive message boards make EVERY car seem like an unreliable POS, simply because there isn't much reason to post unless you are having problems. The Camaro boards and even the Acura TL board make those very reliable (in my experince) cars seem like 1970's AMC products.
So I am assuming that the M5 is going to be a pretty reliable car, with proper proper maintenance. I know that cars cost money, and eclectic cars like M5's cost more money to own and drive. Replacing the self leveling in the rear, new water pump, new radiator, new A/C, and periodic valve adjusments are no big deal to me, and I expect to have to to that sort of thing on a 15 year old car no matter how low the miles.
So, given proper maintenace and upkeep, I think an E34 M5 can be a reliable daily trasport and road trip machine. Am I right? Just in case I'm wrong, I'll be keeping my 99 Z28 Camaro for my "new" car ;-)