I played with the Kong Duck for a while; I much prefer it to the Ropeman 1, largely because it's got smoother teeth that are less likely to chew up the rope or have a rope failure in the event of a fall, and it's rated for 8mm rope and I'm using Oplux for tether and lineman's belts.
I thought the duck was neat, but got rid of it when I decided to go with tended prusiks instead: I'm using the ottolini mini figure 8 with a small loop of paracord hooked into the carabiner as a tender: it's definitely not as smooth as the duck, but adequate.
Some people have asserted that the duck isn't rated for the sorts of uses we're putting it to, but I did some research and disagree with that assessment; caveat that I'm in no way an expert. Specifically, folks point to the duck in a load-hauling configuration where the rope is supposed to go around the central carabiner connection (for example, in this video:
www.youtube.com/watch?v=At3m_DDP2fA). But critically, that is for load lifting, and has less to do with breaking strength of the duck and more to do with not accidentally unlocking the load. This video from Kong shows this in more detail:
Because of the shape of the Duck, the oval carabiner will move around more freely but isn't strictly necessary. I tried it with an oval and at one point realized that I was cross-loading the carabiner on my lineman's loop, just through natural movement. I noticed before I put a lot of weight on it, but still; that was one of the reasons I moved away from the mechanical ascender in general.