On field performance is always first, but the combine is always a balance to the production. The problem comes when one is weighted too highly over the other.
Looking too much at the combine, not enough at the tape, can get you a Darrius Heyward Bey or Phillip Dorsett. Looking too much at the tape, not paying enough attention to the measurables can get you Peter Warrick or Mike Williams.
Big mistakes are made both ways all the time - it's such an inexact science.
The top guys coming in to the combine usually remain the top guys, unless they have awful workouts.
Where the big difference comes in is you might have 5-6 guys at a position rated pretty similarly, somewhere in that 3rd/4th round range. Each have some questions marks about them. A great workout can propel a 3rd/4th to a solid 2nd round grade, and an awful workout can then drop a guy to a 5th/6th. If there's guys GM's aren't quite sold on, the workout can make a huge difference.