Well, first off, note that I'm not too concerned about the finances. For me, I'm just looking to invest or donate to projects that help advance solar. The fact that it had a return that gets credited to your utility account was just a bonus for me.
My main concern is mostly about intent. Do the people behind the program really want to advance solar, and that is why they are doing it? Or do they not really care, other than to find a way to leverage money from people? That can be hard to determine. I was encouraged by Arcadia's link with Sierra Club as a partner. I even tried to contact Sierra Club to ask them about their partnership with Arcadia, but so far have not received a reply. I'm not totally sold on Arcadia yet. I like how the program is structured, but I'm still not confident they have the benevolent intent I'm looking for.
BTW, they recently changed their offering. It used to only be $300 increments, which is consistent with one solar panel. I guess they decided to break it into smaller chunks to make it more accessible. I assume that means you sort of share panels (conceptually).