I've used the Michigan reservation system many times and never experienced a problem, but then again, I use the online version. A couple of years ago while on a trip to the U.P. we had to cancel at Indian Lake SP due to a death of a close friend. We used the phone system and it went well.
I also use the Florida online reservation system and while it's different it works well. I just wish I could get into some of the more popular FL parks during the winter months. In addition, I've used an online system for Kentucky parks and again, no problem.
It's a bit concerning to hear of some of the problems people report. I guess the best thing that can be said is that a great majority of campers have no problem reserving campsites.