In the Texans forum, I've seen a number of people point to PFF data to make points about players. On my Chronicle blog, there was a minor debate the other day in the Dunta Robinson post about the validity of their data. On whether people can actually chart the things they claim they can chart. So I did a follow up blog post: Pro Football Focus: How Do They Put Their Numbers Together? They talk about the limitations of the data, and the Football Outsider people further discuss why the PFF method for grading players is problematic. The piece also has more stuff on Dunta Robinson in it.