I used Quixx on a small scratch on my driver's door. About an inch long but you could catch your nail on it. After using the Quixx step one and two, it was still there although not as noticable.
Next, I wet sanded lightly with one of the small strips of 3000 grit paper then redid the two other steps. The scratch, while still there, is hardly notcable and if you didn't know where to look it would be very hard to find.
Overall, Quixx worked (IMO) very well. I'm sure with a little more wetsanding, I could have removed even more of the scratch but didn't want to chance taking too much clearcoat off. If the scratch was down the the primer, I doubt Quixx would work but for more shallow scratches, it's certainly worth a try.
For deeper scratches, maybe Langka or Touchup 123 would work better (both of which I've tried but didn't have a lot of luck with) or perhaps a combination of either Langka or Touchup 123 AND Quixx would be the best solution.
LL