I thought Leaf was one of the most physically gifted QBs to come into the league in the last 15 years. I also saw him take a bunch of snaps. If you go back to 1990, get all the film on every stud college QB, watch it all, there is no way Leaf isn't a top 5 QB.
What does this have to do with Rivers? He's an unknown. Despite the film, the measurables, the talent around him. It's still an unknown. If you could project - with ANY accuracy - to the nfl, you wouldn't have Couch taken 1st overall. Or Harrington 3rd overall. Even well paid experts, whose job it is to know this stuff, get it wrong - all the time.
Thanks for the anwser, Sigfawn.

For those of you making Leaf/Harrison/etc comparisons, you might find
this article from FootballOutsiders interesting. It points out that, while it may seem difficult to predict whether a college QB will succeed at the NFL level, there's actually one combination of metrics that's been a remarkably successful predictor in the past: college starts and college completion percentage. Leaf and Harrison and the other busts fared poorly by that combination, and Roethlisberger, for example, did quite well. Pro Football Forecast 2006 will evidently give more details, but apparently the correlation with NFL success over the past ten years has been strong.
No quarterback who started 35+ games and completed 60% of his passes and was drafted in the first two rounds has been a failure. Rivers started 51 games and completed 67% of his passes. As the article states, "According to this projection system, Philip Rivers will emerge as one of the top quarterbacks in the league over the next couple of years."
I don't know how much stock to put into that. Maybe none. It's not the reason I'm high on Rivers, anyway. I'm high on him for the reasons stated in the spotlight, which should appear on the site shortly. The short version is just that Rivers is in a really great situation. QBs in offenses that score a lot of points are always good fantasy options, and the Chargers were top five in the league in points scored in each of the past two seasons. Other first-year starters taking over in successful offenses in the recent past have all done well right away.