There are a handful of things I think all folks should visit at least once in their life. Statue of Liberty (still on the list), Pearl Harbor and Mt. Rushmore.
It's just beautiful. I lived there as a kid and we'd inevitably drive up there when folks visited, and it was okay way back then. In the intervening 15 years or so, they're really updated the entire site and it's gorgeous.
It's really a nice and scenic walk all the way the avenue of the flags and presidents. All nicely done! We're planning a visit again several summers from now. It's nice with kids to spot your own state flag on the way up. It does get hot in the summer time, so wear shorts and a hat - it's sunny as hell most days and it shines off the granite like crazy.
All in all, a fantastic experience. More