IMHO, I think you only need to look at the 1st and 2nd rounds to judge any draft. What happens in rounds 3-7 is meaningful to a football team, but not nearly as much as those 1st two rounds.
The bottom line is either you drafted excellent to elite NFL talent in the 1st two rounds or you failed. The Bills have been a team of average starters and backups for a decade because they failed to draft elite talent in the 1st two rounds. Kyle Williams and Stevie Johnson are not substitutes either. They should be bonuses.
2005: F
2006: F
2007: D
2008: F
2009: C (only because those 3 starters may still be on the rise)
And that explains the Bills situation right there. The lack of elite NFL talent and the Bills failure to recognize it and acquire it.