We went there for the 150th reenactments, and although artk2002 was right (it was brutally hot) it was still a spectacle to behold. Because it was the 150th, the number of reenactors there far exceeded the usual count, and the battles we watched were much more engaging because of it. Seeing hundreds of cavalry clash, and thousands of soldiers shooting it out, while dozens of cannon were blasting away, was something I'll never forget.
And yeah, Suttler's Village (the area where all of the merchants set up) was really cool too.
It sounds like the weather conditions were pretty accurate to the battle, then
; if I recall my reading correctly it was very hot then, too - and the soldiers were in wool uniforms! My DH and I visited 5 years ago, right after Labor Day weekend, and it was really nice. We hired one of the personal guides (VERY well worth the money), and then did some exploring on our own. It really wasn't crowded at that point, so it was pretty easy to get around. Seeing all the different memorials was interesting in its own right, and being there really gave a feel for how things were.