Lots of beer and fun as long as you have no problem with overcrowded beer tents. If you want to get into one of the several beer tents, you have to either reserve seats or be there early because (esp. on weekends) they close very early due to reaching their "seating capacity" (on weekends some tents "max out" at 10am).
Some brawls break out from time to time, but security is pretty good and it's rather rare to see real fights break out despite all the alcohol.
Other than that it's a fun atmosphere and a really mixed crowd (many different nations and of almost any age).
Like every famous tourist attraction, you should also expect quite hefty prices, I think this year ~8.50 EUR (@12.50 USD) for a beer (1 litre) or entrance fees somewhere between 5-10EUR for the rides and other attractions, they offer.
They open at 9am on weekends (10am during the week), from past visits I would expect the first tents being filled to the max by 10am on Saturday, but normally you should still find a couple of tents, who aren't closed yet, up to 11am.
After 11am you will probably have a hard time on weekends, after 12am you should expect being shut out by security without a reservation. But even then you will be able to enjoy yourself, if the weather is ok.
Have fun and enjoy your time in Munich
