Biked in sections of the trail between Custer and north of Rochford last October with my son doing self-supported tour through Badlands and the Black Hills. Was just beautiful though somewhat cold. Be aware that in part of August the hills will be full of motor cyclists attending races and rally in Sturgis, SD.
Don’t know if you plan to stay in Deadwood but it might be hard to get a room unless you register early. I have stayed in the ThunderCove Inn in Deadwood on 3 different times and felt it was a nice place and reasonably priced. However it was offseason during January. Here is a link with reviews. http://www.tripadvisor.com/Hotel_Review-g54578-d1901813-Reviews-Thunder_Cove_Inn_Suites-Deadwood_South_Dakota.html
Last fall stayed in the Super 8 in Hill City and I found it was also a good place to stay. My son was riding his road touring bike and I was riding a loaded recumbent LowRacer. Only a couple of sections did I have any trouble with loose sand and gravel. Don’t know what other sections are like, might be some less packed trails when you go south of Custer.
Here are 2 videos my son took while we were on the trail.
http://www.youtube.com/watch?v=tYuRzAcEnp4
http://www.youtube.com/watch?v=INw5XqjxflA&feature=relmfu

Lots of bridges and tunnels between Hill City and Rochford. Slope never runs above 4%