The Mount Rushmore monument itself is a bit disappointing to say the least....however :::::
In 1994, I had a holiday of a lifetime and drove Route 66 from Chicago to LA. I drove back to Boston via the northern route, taking in South Dakota. It was easily the best trip I have ever taken. I just rented a car for 2 months and took off. You don't really see much of the real America from the window of a bus, so driving myself was quite an experience. I saw some wonderful things that Autumn, and drove over 7000 miles in the process ! Small town America is wonderful. The people are so welcoming, especially off the beaten track, as they don't get to see many Brits. Cheap Motels and the best breakfasts in the world !
Anyway, back to Mt Rushmore. Its disappointing in the respect that you can't really get very close to it. The Visitors Centre is about a mile away, perhaps more, so the sculptured rock faces appear very small. Forget all about "North by Northwest" Hitchcock has a lot to answer for ! If it was in Switzerland than there would be a cable car up the side and a nice revolving restaurant at the top ! However, beware making any negative comments when yanks are present, as it seemed to be some kind of pilgrimage for most of them.
But the area of the Black Hills is well worth visiting. I stayed in Keystone, which is like an old Western town. Custer State Park is wonderful, with lots of buffalo, and the Needles Highway was fun, although not recommended if you have a RV !( see link below ) Keystone also has an old railroad, with some restored steam trains, just like the ones you see in Westerns.
The Crazy Horse Monument is just up the road from Keystone. The whole are of the Black Hills was and still is a sacred area to the Sioux tribe and there are places that you can see how they used to live.
So, all in all, well worth a visit and you might as well see Mt Rushmore when you are there !
Hopefully the following will be of some interest ::
http://www.travelsd.com/Attractions/State-Parks/Custer-State-Park
http://en.wikipedia.org/wiki/South_Dakota_Highway_87
http://en.wikipedia.org/wiki/South_Dakota_Highway_87