This September, we are going to Cherry Creek for the 3rd. consecutive year. What TexasTom and Bumps have described is in my opinion exactly how it is. The road from the East (Parker to the entry booth) is quite long so you could have a long wait. Our son lives in Denver and he has stated that when its full, it's full and you wait until someone leaves. There is a sign on the booth pretty much telling you the same thing. We have been going in September and October, arriving around noon on Friday for check in so we have not a had a problem. During the dead of the summer I'm sure it is different. I've never left during the weekend daytime hours so I've never had a problem, but then I don't want to have one either. People are actually lined up at noon to register at the information/registration center. Without a doubt it is very attractive recreational area. We got reservations in March, which was the earliest we could get for this September, and we did not get were we wanted. Cherry Creek is a really nice peaceful, pleasant place to camp. Here a couple of pics I've taken.
Those are pelicans (yes) in the center picture.