I was just looking at one of those the other day. I am still undecided. I think it depends on the temperent of the cat. Elliott freaks out when she goes outside, so I could see her struggling to get away and maybe even jumping out. On the other hand my first cat was extremley mellow and I don't really see him struggling or freaking out at all. The one that I looked at had a a short (about 5 inches) chain that hooked on to the carrier and then on to the cats collar/harness.
If I knew my cat better (only had her about 1.5 months) I would know whether or not I was comfortable w/ them. I am really interested in them though