Interesting that you should say so, because Peter does:
Bear in mind that our Lord's patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction.
~2 Peter 3:15-16
He says that Sha'ul's writings are Scripture, and I agree with him. However, the Jews have a slightly different view of canonicity than we do. To most Christians, something is simply either canon or its not. To a Jew, all the Tanakh (and to a Messianic Jew, all the NT as well) is inspired and canon, but there are still differing levels of authority, with the Torah at the top, then the prophets, then the writings. The Torah, being the highest authority, was the plumb line to which every other Scripture must conform. Since Yeshua is the Living Torah, I put His words, even recorded by others, on the same level.
In regards to Sha'ul's writings, I regard them as Divinely-inspired commentary, since they are completely dependent on the Torah and the Gospel and simply seek to explain them. There are only a handful of cases where Sha'ul is revealing something new that he received by special revelation, and he's careful to identify those (and salvation by grace is not among them--that's from the Torah).
Since Sha'ul's writings are completely dependent on the Torah and Yeshua, they fall under the latters' authority, so they are not on the same plain. In terms of literary value, one must also rate the Torah and Yeshua higher for the same reason.
Again, that's not to denigrate Sha'ul--it's to elevate God Himself.