I've experimented with this too.
I like a pneumatic form for the even distribution of pressure, but I think it's somewhat limiting. Also, they tend to be massive and I have only a small shop. I was debating using 1/8" (3mm) coated steel cable in place of the rope to avoid issues with stretching. Never got that far though.
I know I'm the odd man out here, but I honestly prefer clamps. The only downside to clamps is tightening them enough to get a good joint, but not so much that I squeeze all the glue out or dimple the fiberglass. I have it pretty well worked out now, but it took a while to get there.
Currently, I'm experimenting with using 1/8" steel pressure strips in place of the aluminum I currently use. This, coupled with C clamps modified by welding a nut in place of the cross handle will allow me to use a torque wrench to set them exactly the same without dimpling the fiberglass if I get carried away.
I'll let you guys know how it works out.