Many people don't like Kiper but he's like a walking encyclopedia. I'm amazed when it gets down to the 7th round and he rattles off a couple minute analysis on those players.
He was ridiculed but he was right on the much talked about opinion of Trent Dilfer over Trev Alberts, even though Dilfer's career was pretty mediocre. Alberts was a major bust.
I found a website called http://www.coldhardfootballfacts.com which has a statistical analysis of correct picks in past Mock Drafts of the 1st round by Kiper, OurLads, McShay, etc. As others have mentioned, quantifying the results is pretty much nonsense as getting the correct picks by position is an impossible task (the top guys got about 25% right). I'd rather be able to review past analysis like: "This linebacker will excel in the 3-4" and see how that player turned out 5 years later. Then one could develop an opinion like "Mayock was right on about half the time" and have confidence the guy knows what he's talking about.
The analysis/forecast for QBs would be very interesting to look back on as that's an extremely hard position to judge.