PFF is used by Jackson in most of his articles, which everyone loves, as well as every media everyone posts links to. It is the new, and accepted, site to rank and grade players. There is no better, as far as I know, but IF we have a better source, please share it.
Your own eyes? PFF's numbers are useful as I said, but just because PFF says something doesn't make it true. It can raise new questions or answer old ones but because PFF says something does not make it the truth.
I mean, it's not hard to watch somebody play and see if they struggled or not.