That's a Rixen and Kaul KLICKfix attachment system - very easy to get on and off, I've got the same system on a different pannier bag. Just press down on the red button whilst pulling up on the rubber handle, one-handed no problem, grab and go. No fiddling around at all.

I can't see any fittings for a shoulder strap on those panniers though (mine has little d-rings in the corners for it to attach to). Nor does it look like the compression straps can be used as a shoulder strap as I believe is possible with Ortleibs. Worth an e-mail to REI to check, perhaps?

EDIT: Forgot to mention, the hooks that hold onto your rack can be used for any diameter tubing by way of little plastic shims, which have three positions - one way around, the other way, and removed, all of which give different diameters. I've found it to fit much better on a 'layered' rack as legot73 puts it than the fitting system on my carradice bag, which is older Rixen and Kaul and relies on each hook being able to wrap around each rack tube, which doesn't work if there're two tubes on top of one another.