The G7 doesn't work well on my Faith guitars - it jumps away for some reason. however it works okay on my Yamaha. the nashville I didn't like at all - not strong enough. I think I gave it away.

The Kyser does put the strings out of tune, but at least it holds the strings down. You don't suddenly finds certain strings are muted like you do with the nashville.

The Shubb is the best of the lot - although it takes time to adjust.

the chinese copies of shubb for four quid on e bay are not as reliable as the original.

All in all - its tricky. Some capos work with some guitars and the faults emerge at a later date - you just have to bite the bullet and pay your money and hope for the best.