I'll definitely be going, it's my second favourite weekend of the year (the first being the Goodwood Revival Meeting). As I live near Goodwood I usually go on all 3 days but it's still not enough!
Friday always used to be less crowded, but everyone seems to have got wise to that now.
Also the F1 teams tend not to do much running on the Friday (must be force of habit) so if you want to see recent F1 cars in action Saturday or Sunday would be best.
Whichever day you go, it's worth getting there early and staying late if you want to get a good look at the cars in the paddock. This also helps avoid the inevitable traffic jams in and out of the car parks ...
The Startline grandstand (or the spectator area in front of it) is good, as you get a lot of noise and tyre smoke, but I tend to go for the Park grandstand opposite Goodwood House, as it gives the best views of the cars against a fantastic backdrop. Plus the F1 drivers tend to stop here and perform burnouts and doughnuts ... If you get time (and it's not raining) it's worth walking all the way up the hill, stopping to watch at various points on the way. There's also a rally stage in the woods at the top of the hill, which this year will be run in memory of Richard Burns and Michael Park.
Keep an eye out for the air displays as well - hopefully the Boeing 747 will be back this year ...
The only drawback with the Festival is that it's become so popular that you don't get the same kind of access to the drivers as in the past. A few years back I actually walked in with Stirling Moss, but I can't imagine that would happen now. The current F1 drivers usually need a police escort to get them through the crowds!
Oh, and take plenty of spending money. There are numerous trade stands and you will see lots of things you'll want to buy ...