As a skins fan, obviously, I never want to admit when a Cowboys player is good. But let me just answer this question for you...
There is, I've noticed, in each "fandom" many mini-fandoms in that. Some are endorsed by all (Newman, Ware and Romo) and some are hated by all (Spears and Carpenter come to mind immediately). However, a good majority of these mini-fandoms are just like their larger franchise fandoms (I'm really beginning to hate the word, I just can't think of a better one, so fandom = FD) in that you have the diehards who make up a small portion of the general populace. They will back the players until they are off the team, and maybe even further (AdamJT13 comes immediately to mind when I think of Roy). There are others who are firmly against the players and would take a LOT of convincing (that frankly will never happen) to get them to admit they are a good player. A good majority though are the bandwagon fans, they are with the players when they are hot, and against them when they are cold.
However, the debate isn't going away. If he has a horrible season, there will be excuses. If he has a great season, there will be excuses because there are just too many ways to view the same play and, as AdamJT13 points out, we never really know what's supposed to be happening in the secondary. You will probably get everyone saying yes, we will admit we were wrong and you were right, but in the end, there is a small chance that Roy Williams will ever acheive a season where nobody can say he wasn't a stellar player.