I did it starting from Deadwood in the 2nd week of June 2007. There had been rain before we started and there were some muddy sections along the grade out of Deadwood. It is gradual but a steady climb for the first few miles from Deadwood. We also hit sections of new gravel that were soft but that probably doesn't happen that often. I had 38c tires and they worked well for the trail. I wouldn't go any narrower than 32c.

Prepare for a wide range in temps and changeable weather. We had some mornings of 40 degrees F or less and lost a day of riding because of 30-40 mph headwind and a high temp in the low 40's.
We didn't make it past Pringle and it was mostly open prairie in that area and not as nice. Custer and north seemed the most scenic. A loop up Spearfish canyon and back to Deadwood was a nice day ride and I would recommend it. We biked back on roads instead of the trail and it was a nice change. Needles Hwy was avoided - scenic but very narrow with a lot of climbs and dangerous on a bike.

Definitely a nice ride and the valleys and hills are very pretty and green in June. It is someplace I would ride again.