I have one and I use it when clients need deeper work. It is okay. Better than just using my thumb unprotected, but I feel it makes me want to give more pressure than I could without the thumbsaver, so I end up with a sore thumb if I over do it. I have the blue one, which is probably too large for me, but I figure it is better than not using anything.
Clients rarely notice it, they think it is either a thumb or an elbow.