Everybody sure does like to make Castonzo their oline whipping boy, and it's so unfounded. People act like he should be on Joe Thomas' level, but honestly, there's only one Joe Thomas. Yeah Castonzo gets beat sometimes and he's not going to be an all pro, but the dude is solid. He is our best lineman and has been for 6 years.
The reason people put Kelly and Mewhort ahead of him is because they don't get beat as much in pass protection, which is how it tends to work when you're double teaming some 300 pound DT on an interior rush versus being on an island for outside speed rushers. Plus AC is always available. If he were ever hurt for an extended period of time you'd see what bad LT play looks like.
Right now, AC is rated 14th out of all tackles in the league. Not just Left Tackles, but ALL tackles. That firmly puts him in the better half of the league, possibly in the top 25% among LTs. Also, keep in mind he's playing next to Vuj, who is doing him no favors and the play calling often makes his job even harder.
He is the least of our problems on the oline.