It works very well on my 12-string acoustic, no buzz and no pressing too strongly to go out of tune. The rubber material seems really good, not too soft, not too hard. The build quality is very good. However, there are other products that are much more easy to handle with one hand, so if this is important for you, try some other brands and construction styles as well.
Overall, I like it but I seldom use the 12 string and I do not have to change the capo position between songs, so the minor handling issues are not a problem for me.
Shubb capos are great. I don't know why anyone would want to use anything else. They are compact, minimising interference when up close and the grip can be adjusted to best suit the position and to minimise distorting the string tuning. And, they are strong and never break or fall off halfway through a performance. I have used Shubb's S1 standard guitar capos on 12 string acoustic guitars for years (25) and they work great and have done the job well but the C3 12 string capo gives a little more extra length which allows you to use the capo more effectively especially up at the 5th fret and beyond, as the neck gets wider. So the C3 is really what you want to buy if you only play 12 string and only want to buy one capo. But, if you can afford 2 capos then get the C3 and the S1 capo. Then you'll never need to buy another capo in your life, unless you lose them or some jealous monkey steals them.