I went the last two years. Its a pretty good time. At night they always have a street concert, and the street is FULL of people. They also have 3 different poker runs during the weekend. I think this year the had a total of 750 registerd cars, and then there are probably another 300 cruising around that are not registerd.
If you guys come down, and you have a little extra time (and good brakes) I recommend taking your car down 16A (needles highway) by Mount Rushmore.
http://www.mapquest.com/maps?city=Ra...:::::f:EN:M:/e
Oh yea, If you need a place to stay, check out craigslist. There are cabins you can rent close to Deadwood. My wife and I went up with 3 other couples and rented a house one block off main.
https://www.youtube.com/watch?v=ysgXCau874k