Well, for some reason I hadn’t thought of using curves before… but now that you mention it, it’s a really good technique. I do exactly the same, except I don’t use curvers; I model the main lines of the car and then, well, rather than snap them, I just fill in with mesh. However, I must say it is complicated to get a smooth mesh, with quads that are roughly the same size and all of the positioned in the right place and facing the right angle… I guess it’s just a matter of feeling it. But, as I said, what you’ve done is really good… I see you do understand topology well.
One thing I’d like to see is the wireframe without the sub-surf thing, however-it’s-called, what I mean is that I want to see where your vertices really are, not where they were moved to by the subsurf modifier… Also, why are you using sharp edges with subsurf? Just curious.
As far as I can see right now, I’d only check the rear-most part of the side, where it joins with the rear… I can see some inconsistencies there.
Oh, and about my cars being of exceptional quality… well, I wouldn’t say so myself… maybe the Skyline, but that’d be it.