I have the canon filter adapter [FA-DC58B] and it works well though my circular polarizer is stuck on it at the moment. Must have gotten grit/sand in the threading when in Australia. I ordered a filter wrench to get it off since polarizers spin when trying other methods. What I like about it is when your filter is on you can use a canon 58mm lens cover, just leave the thing on. It extends when the lens extends and then collapses again when the camera is turned off. I kept it on the entire time I was on vacation with exception to using it on my DSLR. Polarizer on the G12 is like magic. *I had looked into the lensmate but decided against it. First because it's like a standard filter adapter and reminded me of a hat brim on the camera and secondly I didn't have time to check one out before vacation anyway so I got the Canon one. It was not disappointing.
The G12 has a ring on it around the lens, there is a button release [bottom right of the lens when facing front of camera] and you screw it counter or anti-clockwise. Then you line up the adapter dot, it has the typical canon silvery-white dot line up marker [the camera just as a little dot indentation to the right of the lens as you face it], and give it a turn clockwise onto the threads and it locks on. You can actually extend the lens through the adapter when there is no filter on but when there is a filter on it the adapter is spring loaded, it extends with the camera lens extending.