Although I haven't used this blade for much yet, I did test it and it appears to be working just fine. It is true that you shouldn't return the blade over the cut a second time as it does not follow exactly.
I have had my scallop blade for over a year now and I love it! It was tough to get it working at first, and I believe I adjusted the tension with the metal knob on the side. It did take some getting used to, but now I use it all the time.
I have found that if I hold the paper tightly under the clear ruler guide, I can reverse the blade to go through a second time if it didn't cut all the way through on the first pass. Of course this means not moving the paper before the second pass.
I would also recommend to anyone having difficulty with this to place the paper under the guide/ruler with the hinge at the bottom (or closer to you) and start with the blade at the top & pull towards you. This works best for me. (your mileage may vary)