Interesting - there are many mushroom houses near me, a major packaging company has their HQ here. But most of them are built into the hillside with only a northern exposure.
There are also several large solar farms in my area, where they've taken fields and filled them with rows and rows of solar panels. I haven't seen any specs on their energy production though. The one is located at an existing trash to steam generating station, they had a large open land area which now hosts a large array of solar panels.
That uses up a lot of land area though, and since the frames are right on the ground (holding the panels at an angle), there's no other use for the space. Any large elevated surface is going to be the best, since the space is currently being unused, and in the case of it being a roof, is simply heating up the space below it when it could be generating power if covered with solar panels. All these silly ideas like the road sucking up money only fuel the anti-solar people who can (rightfully) point to such nonsense as a huge waste of money. A REAL breakthrough, and I'm sure this is being worked on, is a solar power generating material that can replace the traditional shingle roof. Not panels mounted to an existing roof - the entire roof covering is solar cells. Get it cost-comparative to shingles and you'd have to live in the woods or be totally nuts to not do it. For a house like mine, it would be half and half. My house is aligned almost exactly east-west, so I have one roof sloped facing south, and the other side faces north. Be nearly pointless to put solar power collection on the north side roof, but the south side gets sun all day, every day, summer or winter.