These are really only useful for very thick or long cables. 6-metre instrument cables just don't coil up into large enough bundles, so you have to feed the strap through the little collar, then wrap it around the bundle several times.
They've never broken or come undone on me, so I can't fault their quality.
However, once I tried the Planet Waves PW-ECT-10, I started using them on all my cables instead.