I've bought one before with my GF. I'm a good 17cm around and it wasn't all that tight. Kinda hard to put it on over the condom but the band stretches quite easily. Unfortunately it also gets quite sticky and gets tangled with yer pubes.

It had little to no effect with us. More of an annoyance than anything. I imagine it would be great for girls who prefer to cum from clitoral stimulation than penetration.