When healthy, Beckett has the ability to break top 10 easily, into the top 5. But, he has rarely been healthy for an extensive period of time. It's unfair to judge his performances, because it's hard as hell to gauge whether he's been healthy or not.
He's got a great fastball, but like said, all major leaguers can sit on any fastball, no matter how good it is. You have to have another good pitch. For Josh, that is a killer 12 to 6 curveball. That overhand curve is just nasty, one of the best in the league. However, since a curve relies on your fingers to provide a tremendous amount of grip (can't find a better word), to rotate off of, Beckett has trouble throwing the curve when his blisters are affecting him. It hurts him. Thus, he just throws fastball after fastball and gets crushed.
I'm going to agree that he'll be top 10. I think he's going to be healthy down the stretch.