I try to go over every year. This year I'm flying over on Thursday night and going to the show on the Friday. As Dave said the Friday is that bit quieter in terms of crowds.

One thing that is definite is that you need to be there early. Gates open at I think 7am, not 100% sure but it does say the time on the ticket. Last year I was at the motorway exit for the event at about 6:15am and there was still a tailback of probably 5 miles at that time.

Certain stuff will get crowded very quickly, the likes of the supercars and the formula 1 cars will be mobbed after an hour or two. It helps to plan out what you want to see especially if you're attending for two days. All the cars will climb the hill twice each day and being at the top of the hill where the rally stage is normally a good spot for seeing the drivers and the doughnuts/burnouts. Be prepared for lots of walking as well as the hill itself is pretty steep with some nice views from the top.

Here's a link to my thread from last year with some photos

http://www.bmwhaus.ie/forum/showthread.php?tid=3638