I just picked up a Keeley and am pretty stunned by it. I use for light compression before the amp and after my overdrive (this is the Robert recommends it, but try it other ways as well) and it'll smooth out the tone without killing the dynamics. Used this way you can dial back or lighten your touch and it'll respond nicely. It's dead quiet. As far as sustain goes, I use it at ~10 o'clock, but even there, it supports the sustain well. Don't really use it dialed up so I can't comment there. If true bypass is important to you, it's there, though mine pops a bit when switched.
Robert's a good guy to deal with. He also has a $ back guarantee, so you could order one, and if you don't like it, send it back within 2 day and get your cash back (though you'd be out some shipping), but you'll have to wait about two months to get one. But you knew all that.
Haven't used the Carl Martin so I can't give you a comparision.