I have a 6006 Mark I. It's been a workhorse for years, not a single problem. I decided to get an extra battery from B&H ($125!), just in case, but really, the original battery still lasts for dozens of rolls between charges. Perhaps I'm lucky, I don't know. The camera system is heavy, a big drawback for me, though it can make some beautiful images. I don't usually rely on in-camera metering, since I prefer incident metering or Sunny 16, but the big Rollei's meter is very reliable, predictable, and accurate. I've shot many slides with it, and as long as I increase exposures for backlit situations, it's nearly perfect.
Worst downside? Good luck finding lenses in usable focal lengths for a reasonable price. Hasselblads are much better for lenses and accessories availability and price.
While I like my 6006 a lot, and use it fairly often--95% of the time on a tripod--I don't love it like I love my Autocord.