Never tried them and don't know about general performance. But they work differently: with the C2 closed system, you can adjust the brake pads' position thus the biting point with a little wheel on the levers which gives the brake fluid more or less room. The downside is that the expansion of the fluid generated by heat also leads to the brake pads coming closer to the rotor, so you have to adjust them in that situation. The O2 open system keeps the pads and thus biting point steady in one position, but you can't adjust it.