I've been a few times, and I guess it depends on how much time you have and what you are looking for. In my opinion, I think that two of the best days are the Sunday and Monday (Labor Day) sessions. By this time, there usually are a few upsets and underdogs still making noise. I've always had tickets to Ashe, but the idea of roaming around to Louis Armstrong and the Grandstand is a lot of fun. And you can make a weekend out of it and enjoy NYC if you have the time. The evening sessions for the Quarterfinals are also a lot of fun.
As for the ticket timing....I always bought tickets about a week before the matches, but I wouldn't hold off as I think they're going at a much quicker rate. The last match I saw live was the Agassi/Blake in the 2005 Quarterfinals...awesome match and atmosphere but, as you can see, that was awhile ago so the ticket timing may have changed.
I really don't think you can go wrong as long as the weather is good...it is a great atmosphere and overall experience.