My roof is metal over shingles and no problems with that aspect. I had it installed after Ivan threw a tree limb thru the roof, punching a 9"-10" hole in it.
We always wanted a Metal Roof anyways so it worked out, we just had crappy installer and since I lived in Panama City at the time, we werent there for inspections but heck, I didn't know squat about roofing to begin with so I wouldn't have known what to look for.
My only complaint is the screws backing out. Several times since the Metal roof was installed in 2005, I've had to replace the screws with larger diameter ones.
Finally having had enough of the leaks this past spring i researched and found the problem and solutions.
First make sure your screws has the 1 piece bonnet that is molded with the screw head and not the "washer" type.
Be certain that screw length will completely penetrate thru the Purling and into the decking.
Make sure the screws are Woodgrip for wood and Tek for metal. I used the WoodZak type screws, they are a bit more expensive but well worth it.
I would definitely have inspector climb onto the roof and look at it or if you're not using an Inspector, then you get on the roof and look at the screws and washers. Look for cracked rubber washers or over expanded washers. Look for screws that are backed out or screws that have obviously been replaced and/or have sealant smeared on the screw heads. I used sealant like this in the effort to solve the screws backing out and to stop leaks.
As for fading, mine has faded somewhat in these 13 years but not bad at all and still looks good.
Good Luck with the House hunting!