I have the 12" and 17" Olfa. Love to take the 12" to classes. I won the 6" Fiskars in a drawing and been using it, definitely not as good as the Olfa.
The Roundabout sounds interesting. I just watched both videos on their website and noticed a discrepancy: the first video has a larger mat than what is apparently available now. The one in the first video can size a 12" block while the current and 2nd video show a 10" block. I think I would be more interested if the larger mat were available, until then I think I will stick to my Olfas and the Fiskar.
I have bought the zip binding system from Martelli and ended up not thrilled. It looked great at the show and then I had difficulty using them at home and got rid of it. I also tried the cutters at the show and they didn't work for me. I prefer my Olfa ergonomic cutters. That said, I do see some other toys I might like. For instance, the lightbox and the kwikspin thread dispensers.