I actually just reviewed these for an online website last month.
I used it on Reiker's SS1 and it didn't interfere with anything, but like Crunchier said in a weeks worth of time of them on the seat, I uesd them maybe three times? Oh and I make at least 20 trips with the kids in/out of their seats M-Th, so it's not like I didn't have the opportunity to use them. They were just too fiddly for me to use...and if parents aren't going to take the time to flatten the straps out, you can guarantee they aren't gonna take the time to fiddle with those things.
However, I did NOT like the concept in using them with a typical convertible seat. There is a good possibility that the harness/adjuster strap will get caught on the cord, or the black plastic do-dad to shorten the strap.
It's basically two pieces of flexible metal with nylon webbing, with a nylon cord in between. The hooks are plastic.