Um, not so.
The C/Y Adapters sold by CameraQuest, by Bob Shell, and by Zörk are all "dead" adapters -- no glass element, just a metal ring.
I asked Stephen Gandy specifically about infinity focus just recently, and he assured me that it does indeed work. I don't think he'd say that without believing it, and he's definitely one of the greatest photographic gearhead cobblers out there in the world. Likewise Zörk promises infinity focus, and I doubt they're throw away their optical reuptation for the sake of an obscure $200 part.
Stop-down restrictions, yeah. But sturdier, sharper, and easier for manual focus than the Canon equivalents (for the 50mm and 28mm lengths, they're rated by photodo as sharper than Canon, Nikon, or Leica -- pretty consistent with my experience with the G2 and (long ago) RTS).
(In fact when I started processing the first roll shot with my G2, I could see the extra "snap" right there on the negatives, before they were even dry, as compared to my Canon-lens negs in the same batch.)
Canon FD lenses on Canon EOS -- yeah, those won't focus to infinity. Canon made a camera that can use most everyone's lenses but their own. Go figure.