I installed a set of flaps on my '11 Wagon last month. Your request for a 0-10 rating depends on how mechanically inclined you are... On my scale, I'd rate it about a 3. The most difficult thing for me is getting the plastic body panel fasteners to disengage without breaking them or the fasteners. All of them were actually pretty easy, but I hate pulling and tugging on plastic pieces, I've had some crack (on previous cars) in the past. None did on the subie, though. I think it took me about 90 minutes to do all four.
Removing the wheels isn't necessary, but it would definately make it less of a pain. Screwing the mounting screws in with the wheels on, particularly the rear wheels, is a pain. I have a very stubby screwdriver, but it was still irritating. While I was doing it I found myself wishing that I'd simply waited until I was swapping over to snow tires next month and would have the wheels off anyway.
I emailed rallyarmor and suggested that they switch to an allen head bolt. They emailed back promptly and explained that the allen head screws have a taller profile and that they are trying to maintain as much clearnance as possible for owners with big wheels and lowered cars. I still think that they could find an allen head with a fairly low profile (rounded) and the installation would be easier and cleaner. They also sell an 'offset screwdriver' on their website, but I've used those in the past and find that they don't work well. Too hard to keep downward pressure on the screw when you are turning it. This is what allen heads are made for!
But I'm just complaining about little details. Over all it is a well engineered kit and pretty easy, if a bit involved, to put on. The written instructions that come with the flaps are good, and reviewing the video helps a lot too.