I second what RobKat said. I went for the first time last year and the grounds pass seems to be the way to go. If you can get there early you can get great seats to the match of your choice (except for Ashe). The hard part is deciding which match to watch. You can end up as close to the players as you would at a high school match. Thats when you appreciate how great these players are. The first few days on Ashe seem to be mostly blowouts anyway. Top seeded players strafing qualifiers and very little atmosphere.
I went to three days of qualifying directly preceeding the tournament and that was outstanding also. You can see a lot of players who are rising in the rankings and some veterans returning from injury. I watched Karlovic, Tursunov, Enqvist, Mahut, Kirilenko and many others FOR FREE. The entire qualies are no charge. Again, the hardest part is deciding where you want to be.
A warning about the food prices on site. Everything inside seemed to me to be massive price gouging. They charge $4.50 for water that you can buy anywhere else in America for $1.50 tops. The food was just as bad. I refused to buy anything just on principle. I brought pepperoni sticks and small snacks I could keep in my pocket. Plus, sit-down meals take time away from watching the tennis.
But other than that it was a great time and I regret that I waited so long before going.