I'm gonna have to give a couple of examples for these... Recent history, since I didn't start analyzing games as closely as I do now until about 7 to 10 years ago. Prior to that I was just a crazy fan.
Underrated: Scott Frost was definitely underappreciated. Tough, fast, but not better than Crouch, IMO.
Dahrran Diedrick was underrated. If our offensive line had not sucked so badly in 2002, he'd have been much better. I mean the guy ran for 1300 yards as a Junior. He missed Crouch badly.
Overrated: Dan Alexander -- the dude was huge and fast but had very little by way of moves. I'll always remember him lowering his shoulder to run over a DB that was already on the ground, and then promptly falling over. And he couldn't catch the damn option pitch.
I also have to go with Tracey Wistrom here. He was a solid TE, but man we kinda fell all over ourselves praising him because he was Grant's little bro. Good, but nothing great.