I've used the Ding King Tool on 2 cars with good results. It was worth the $20. It does take an hour or so to fix a big dent.
Here is Consumer Reports advice:
"Our tests of two kits, Ding King and DentOut, showed that both worked about the same, but the Ding King's lower price makes it a better value. We also liked the adjustable feet on the Ding King's pulling bridge. Either kit is worth trying, although they are limited in use and we found it hard to achieve perfect results.
Experience is important for getting professional results, and our amateur testers had trouble achieving them. If youre willing to settle for a less-than-perfect repair, you might try one of the kits. The investment is relatively small, and if you dont like the results, you still can go to a professional."