Of course fixed bid is fantastic for the client, but it can be a really bad deal for the contractor. I work for a consulting company that does a lot of fixed bid work, and we've been absolutely destroyed by some projects where scope got out of hand. I'm not blaming our clients for this, but I what I'm trying to say is that working on a fixed bid basis has its own set of challenges including things like arguing whether feature X is covered by the original scope or not.