I went last year during my vacation -- it was my first large con. I liked it a lot. I got to play games I had always wanted to, but never had the chance, such as the Buffy/Angel RPG (the GM was a guy who worked in show business and used headshots of real actors for the NPCs - cool!). I also volunteered one morning and got assigned to the computer game section. According to the people I met there, Gen Con SoCal is about half the size of Indy, and so is a much more manageable experience, especially for a beginner like me.
As for advice about what to do:
- If you like card/board games, get some open tickets so you can drop in on a game or two
- go to True Dungeon/True Heroes! It is excessively cool
- check out the auction if you want to pick up some rare (or more common) stuff at a good price
- try out games that look interesting to you. Be careful to keep some buffer time between games because they rarely finish at the time they're supposed to.
- volunteer for a few hours, especially near the beginning of the con. It's less hectic and gives you the opportunity to pick up what the 'buzz' is.