If serves and forehands were all that mattered, he would be top 3 in the world. His backhand and movement, though, are about 100 in the world.
He is a lot like Roddick, but...
JJ has a smoother service motion. His serve is tougher to read which makes up for it having less sheer pace than Roddick's serve, making it pretty much equally potent. He's also taller, which makes his serve tougher to read and allows him to hit more places.
But Roddick's second serve is way better. Roddick is also way way way way more athletic. As far as mobility goes, Roddick is Vince Carter to JJ's Vlade Divac.
Where I think JJ might have the biggest edge on Roddick is his forehand. Roddick can probably hit his forehand harder than anybody, but he has no feel at all. He can bludgeon the ball deep into the corners but he can't play sharp angles, take the pace off an hit really extreme angles, and his motion is relatively easy to read. JJ can put his forehand into more places and his motion is much tougher to read.
Actually, JJ has a lot of feel for a big guy. He has really soft hands at the net, and his drop soft is pretty wicked. If JJ is in fact unbeatable when he's playing well, as Mats claimed at the US Open, he will go down as the most lumbering number 1 perhaps ever.