SXSW has been going on for 20 years as a film and music destination in creative Austin, Texas. When they added an Interactive component, it quickly became a mecca for web developers with over 10,000 professionals attending the event. It's long, it's creative, it blends music, film, and interactive - and it's a great way to get in front of the mavens and connectors of the web development world. Projects get noticed there. From what I've been told, it's exactly the type of event that Andy and I will shine at. (Not IT-Corpo jibber jabber, but rather by and for people who actually get their hands dirty and make things.)
We've never been. We don't do a lot of promotion. We went to OSCON to first launch because it was cheap and local. We've paid for a few banner ads, done some PR work - but that's basically it. Going to SXSW could very much change concrete5 from a "under the radar" word of mouth project to a major player.