I-90 gives you an ideal midway point rest stop city in Sioux Falls, a higher speed limit, the ability to drop into the tourist trap that is the Wall Drug Store (pretty much right off the highway), and, if you need anything before driving into the Hills to Deadwood, Rapid City to get supplies.
Once you're in Deadwood you'll be fairly isolated in the town itself or can roll over into Lead.
Set aside a day to drive all around and check the sites and enjoy the views in the Hills... Crazy Horse, Mt. Rushmore, the little towns, Rapid City, Sturgis, etc.
When I was there in 2005, I played at Saloon #10 (super fun) and Cadillac Jack's (was alright, had the biggest game in town at that time). Didn't go to Silverado or Gold Dust.
Would personally be interested in a trip report of the newest room in town:
http://www.deadwoodlodge.com/casino/...rs-poker-room/