When I was there in the summer, there were traffic jams due to people stopping on the highway to take pictures, and all the walking trails around the hot springs and such were crowded with people. If you don't mind some crowded conditions you may not mind Yellowstone in the summer. It's not my cup of tea.
That being said we went in late September a couple years ago and it was still very crowded. Even the concessionaires commented on how unusual the crowds were for so late in the season. We left Fishing Bridge campground on the day it closed for the season. You just never know.