Here, as on all sports boards, the question is who or what is an accepted source around with to debate an issue. Places like PFF, statistics, front office/coaches' quotes, et. al., are used to support a position, and the rebuttal is often/always to disallow the source. Perhaps someone could point out some objective way to evaluate schemes, teams, players, drafts, etc., so that we could all be on the same page.
I think we just have to strike a balance between "Source X is necessarily the final word" and "I only like Source X when it agrees with me." That, and be OK if you and another poster don't like the same source.
For me, I like PFF. I think many of the arguments against it are flawed. For example, it's regularly leveled against them that they don't know play calls and assignments when grading tape. But people usually don't carry that out to realize that neither do NFL scouts when they scout college prospects or other teams' players. And what's more, it stands to reason that the instances where a player blows an assignment and actually receives a positive grade on the play are extremely rare.
But at the same time, I do see some flaws in their grades on occasion and (more frequently) I see their grades misused. I don't think anyone at PFF thinks their player grades should be conclusions in and of themselves to be considered without heavily factoring other considerations (e.g. scheme, talent, and other matters of context). But that's how many people on the web use them (not making this allegation of Captain, to be clear).
But the grades aside, their statistics are really useful and insightful. In the pantheon of publicly available sources, they're certainly in tier one for me. They're just not alone there.
And finally, while I am far less qualified than many sources on the web, I do look at things myself and form my own opinions. This is not to say that I hold myself as an authority over former scouts. But I don't really see the point in following all of this so closely and watching all these prospects if I'm simply going to decide that I'll agree with everything that a given web site says.