We've had our 2.3kW system for 2 years now, and pretty much have zero bills. We accrue credit over summer, and then use most of it over the winter, but we haven't actually owed them anything. We got the 40c + 8c feed in tariff. We haven't had any maintenence, and don't expect any.

As far as re-sale value goes, I think it's becoming a selling point, especially if the system is big enough to zero your bills. For street appeal, I think the ones that are flat on the roof, not on frames to get the right angle, are much much nicer.

Potential shade from neighbours' houses or trees is really, really important. If they're going to get significant shading, it won't be worth it. If it's partial shade, try to look for the panels which still produce in partial shade. Most types (at least 2 years ago this was the case) don't produce anything if any part of the panels is in shade.

Position wise, my understanding is that north-facing is optimal, east facing is next best, west facing is third option, but not great, cos when panels overheat, production goes right down, so all of the hot afternoon sun overheats them and goes to waste, and south-facing is pretty much pointless.

I recommend the whirlpool online forum for helping you to choose a reputable local company.