I think it's fair to be a little down on Peralta but not excessively so. I don't expect him to be an ace; he's a little too flawed for that to be a reasonable expectation. But I do expect him to be better than average, somewhere around respectable #2 starter level.
And while he has shown his typical flashes of brilliance, he all-too-often falls into those epic innings where he can't find the strike zone, throws non-competitive pitches, and lobs over a meatball. He's a very frustrating pitcher to watch over a full season. He has all the pieces to make it work.