I've only been once, so i'm sure someone will come along with better advice, but...
you can still have a good time, but you likely won't be getting into any tents (especially sans reservation) showing up later in the day. There are still biergartens and such you can enjoy...and plenty of carnival rides. Drunk bumper cars might be one of the greatest things ever. Definitely wear lederhosen. If you don't, you'll be one of the few.