I just put it onto my 2-cell Elzetta, which required removal of the O-Ring. The outside diameter of the ring very closely matches the diameter of the body. The lower profile of the switch boot, compared to the McClicky'd Surefires, makes momentary activation using the Surefire/Rogers technique less than 100% reliable. On the positive side, the added tailcap length of the Elzetta makes it more comfortable. It works very well with a cigar grip, using the base of the thumb as the activation point.
Although the ring is quite thick, there is three pieces of notebook paper's worth of gap when the tailcap is fully tightened. This allows the ring to rotate and wobble slightly, which is practically identical to the fit with the Surefires previously mentioned.
I've only used it with a lanyard for a limited time, but it's safe to say it will work as well as any of my other lanyard rings or clips with lanyard holes.
Grizz