Stats alone are an awful way to judge a player because the NFL, by and large, is not a sport where a player can go out and be a world beater on their own merits alone. Stats can be artificially inflated / deflated by the abilities of the other units / players on the field with the player you're measuring.
If you watch someone play and have an objective bone in your body you'll be able to form a much better picture of what they're really like than you'd be able to by looking at box scores. In this I'm with Mr Scot. I would prefer to judge a player based on what I can see as a long time football fan with some knowledge of the game than by a series of numbers that don't tell you even a portion of the whole story.
If you watch someone play in order to rate them, what are you looking out for? Number of INT? Number of TD? Number of completions? Are those not stats?