Our daughter has been obsessed with it since we took her when she was 10 or so. We try to go at least once a year. We have handicapped tags, so getting there early isn't necessary for us to get close parking. We normally get there about 11 or so in order to watch the parade through town. If your family likes to be up early and see all the production before and as they open, it is fun!
The things to watch for are the scantily clad adults, random role playing folks yelling at you, inappropriate language at times. There are also some crazy costumes! A lot of the extra people dressed up, performing, standing stationary, etc. are paid professionals. They take it all very seriously and stay in character the ENTIRE day!! Some of the 'beggars' and 'vagabonds' sit around and yell obscenities or nonsense at people as they walked by. We just explained to our daughter to just keep walking and ignore them. Most of the time I can't really understand what they are mumbling.
If you check online, you will find a listing of special events. Some of those are really neat. We try to always go the weekend after Thanksgiving while all the crazy people are at the Black Friday deals, we are having a blast at RenFest! Even when it is 'crowded,' there is enough room to move around and feel like you have space.
Check Groupon for discounted tickets!