I use a G7th, sometimes like you said Kenny, it doen't clamp tightly, but I've found if I give it one last gentle squeeze after stickin' her on the neck, she does ok.
That's odd... The way I fit the G7th, I place the pivot point on the high E side and - and here's the trick - I press the capo down at the low Es so that it's fretting those strings before clamping the capo and in effect fretting all strings. Do you follow? Works every time for me.
I like Shubbs, but here's a few alternates. The Planet Waves NS capo is wide enough for 6 or 12, they're cheap and work great. Nice and light.
Paige makes a capo for 12 string that has little bands that depress the octave strings so you can apply equal tension to all strings. These are pretty interesting.
Most capos are pretty cheap accessories (with the exception of the G7 and the Elliots).
I just buy one of every capo I encounter and experiement with them. Differant capos on differant guitars. I'm sure most of you are familiar with the G7. Have any of you encountered the Elliot (also sold as the McKinney) capo. These sell for well over $100 but some players swear by them. Some swear when they hear the price.