Some companies created at SW have gone on to be real companies, but SW is really better for getting the feel for working with other people and then working with them on other projects. I can think of five funded (not that that's a metric for success) companies that grew out of Seattle Startup Weekend relationships. While I'm not seeking a co-founder at the moment, everyone on my co-founder shortlist is someone I met at Startup Weekend 3 years ago or I met directly through someone I met at SW.