The NFL Combine can make a lot of many for NFL prospects, but it can also fool teams into wasting money on players that look good in shorts, but don't play well when they put the pads on. Even the best scouts in the business will get fooled by a fast forty time, so it's important to know what to look for when evaluating talent at the NFL Combine.
Mike Lombardi wrote an excellent article about how to intelligently watch the Combine, and you can find excerpts of it over at Hogs Haven. Here is the real crux of his argument.
Mistakes occur each year when teams believe what they see on the workout tape and neglect what they see in game film -- or they misjudge the competition. Evaluating college players requires an evaluation of the players' opponents. Level of competition is the key factor in evaluation, so when you hear someone say they watched 10 games on an individual player, you might want to ask what 10 games? Or else why did you waste time watching him play against players who will never come close to playing in the NFL?
The Combine is an important means of evaluating talent, but it should not be the only means that teams use when trying to make a draft selection. A forty time in shorts on a Saturday in February is a lot less important than what the player has done during his College career. If you really like what you see from a player by his game film, don't sour on him based on what you see this weekend. Use the information, but trust your instincts.