I had a Hassy lens to Canon EOS mount adapter for a long time. I ended up only using it once, making a decent short tele out of the 80mm Planar.
For me, it was a bit of a waste. I used it for a 'fashion/nude/portrait' session with a friend. It, simply, just felt very limiting. I kept the lens at 2.8 and f4, just so that i didn't have to deal with juggling the viewing versus taking aperture, but the focus screen was dim because of the max 2.8 aperture, and i didn't like feeling constricted to those two apertures. It just wasn't worthwhile. At the time, i didn't have an 85mm lens with me. But as soon as i started using this adapted kludge, i realized i'd rather have just shot it all with the 50mm Canon lens i did have with me.
Granted, there are probably certain applications for which using this kind of adapter has fewer compromises. But, for people, for me... ick. I won't do it again.
I don't believe i ever scanned the final results, but the contacts seemed okay. What meant more, though, is that i didn't really succeed with the images because i was too involved with the equipment. I'm sure if i had been more used to that working process, it wouldn't have been so intrusive, but if you're planning to do this only occasionally, you may find it just gets in the way.
I was working with a high quality adapter - i believe it may have been a Novoflex. I also have a couple of Nikons (FE2 and F100). I don't see any reason why my experience would have been different if i had used one of them, instead of the Canon EOS 3.