September, October can be great months to enjoy riding in the Badlands & Black Hills region. Not much traffic, colors changing, usually can find good deals on lodging (several are closed for the season) Waitress' are done being crabby with tourists! :wink:
As mudbug said, the weather CAN get you. With elevations at 7k & less, you can expect frost in the mornings. Pack everything you can.(Gerbing)
If you can watch the weather & plan your trip accordingly, you should have a great time.