I've been to two "Major League" hackathons (CalHacks and YHack) and really enjoyed my experiences at both as a current undergrad studying CS. Not often do I get the chance to meet 1000+ people my age that are interested in exactly what I'm interested. It's a great time for meeting new people, and maintaining relationships after the weekend is done. I landed an internship this Summer in SF from building a hack and talking to engineers, so I'd say it's been an incredibly valuable experience.