We went in Mid-April, and it was not busy at all! From what I understand, May through September are the busiest times. They have a nice gift shop with a few large informational boards about DTNM. The guide in the Visitor's Center was very knowledgeable about the tower. We also hiked the loop around the tower. It was steep at times, but you don't have to be a pro hiker. Lots of great picture opportunities.
Great Hiking and Experience in general. Devils Tower is out nations first National Monument, and when you visit you will understand why. There isn't extensive hiking, but it is fun with views of the tower, Belle Fourche River, and wildlife along the way. The paved walk near the base is also a nice stroll, and the historic visitors center is always informative.
Even if I had not seen "Close Encounters of the Third Kind" about fifty times I still would have came here. Early September is a wonderful time as it was comfortable, great for that walk around the tower without getting overheated. The path is in good condition and easy on the feet and legs, and offers different viewpoints of the tower. If you are a prairie dog fan this is another good location to watch. Wonderful to experience up close and further away.
A must see. We took an overnight trip from Rapid City. It was an easy drive. Our room had a picture window framing the Tower. Easy hike around the Tower and quite beautiful - every angle has something different to enjoy. There is something for hard core climbers, strong hikers, and even those who need a less strenuous walk.
Beautiful drive and well worth the cost of $15 (or $80 if you buy a NPS annual pass). Devil's Tower was so much more than I had thought it would be. I was absolutely blown away by the Tower and it's surroundings. A definite must-see when in the area.