Sometimes I refer to myself as a transit nerd, but honestly, I don't come anywhere near the ZIP code of the obsessive public transportation wonks over at Seattle Transit Blog. I make a point of riding new routes on the first day of operation whenever I can, but they get down to a granular level that escapes me. They sometimes do math, and even graphs*! What I mean to say is, I can't communicate at their level, but I love to read their blog.
But sometimes their posts should be read by everyone who uses multiple transit options in the Seattle metro area. This post by Zach Shaner is an exhaustive pro/con list about Zipcar and Car2Go, including pricing analysis, availability, insurance costs, and much, much more. He finds that there are a lot of ways the two services complement each other. If you care about carsharing, you should go read the whole thing.