They work great, and it is all I use now.http://robo.bushkillfarms.com/cut-out-frames/
String is not only a pain in the butt to put on, but the bees end up burr combing it up. Rubber bands are a little easier to use, but when you get heavy comb, the bands stretch with the weight and he comb doesn't stay straight.
I use regular frames and rip the pieces on a bandsaw before assembly. I then staple embedding wire on both sides and use a wire crimper to tighten the wires
Next batch I make I'm going to try fishline instead. I have done a bunch of foundation with fishline (credit to fatbeeman) and it seems to work great and is much easier to get tight.
I also use two loops of embedding wire around the bottom bars to act as hinges.