I wouldn't bother if I was in Seattle; their power is already pretty clean. But I'm on the East side so I have been on PSE's green power program since even before I went EV a few years ago. My house is surrounded by trees so I can't do solar (although I have invested in a community solar project).
I didn't actually sign up when they first offered it; I cynically figured they had a program anyway and me paying extra wouldn't make a difference (that is, they wouldn't build more if more people signed up). But then I dug around a little more and found that a non-profit checks their numbers and makes sure that they really build enough capacity the cover the people that sign up. I like that aspect.
The utility is regulated, and has to provide power at the lowest possible cost--without regards for external costs. So paying extra for a program like this was the only way to help clean things up. The extra cost is small, and at some point volume, innovation and better regulations should make this necessary. A couple of years ago several local Roadster owners got to tour one of the wind farms that is part of this program; the employees seemed awfully excited about the great things they were allowed to do with just a little bit of extra money.
Of course, this is all my recollection of a situation I tried to assess as a lay person several years ago. I hope rolo or r.b or somebody with industry ties can step in if things have changed or I misunderstood it.