Granted, Barclay's books are very readable, but I think that his skepticism towards the veracity of the Bible cannon negates much of the apparent value of his work. He believed in the source theory concerning the books of Moses and did not believe that 2 Peter was written by the apostle.
To those of you who point out that his writing is of better quality than those of the faithful steward, I would say that I consider it more important that a writer believes unreservedly that the Bible is inspired of God than any ability he has in displaying good stylistic qualities.