I have no problem with demon characters and have a few myself, actually. As long as the operator of the demon is literate and does not godmod and get TOO unrealistic, then I see no problem. Now, those that come up all "kills ur cub no miss eats it" are the kind that I totally hate. Those and the ones that take things too far. I'll give an example.
I was in an RP hanging out in a den because my demon wolfess charrie can't be in the sun and she was resting there and there was a lion pride there. They let her stay as long as she didn't hurt any of the cubs and one of their kind thought it would be fun to be a demon too so went out and "got bitten" and started the change. Well, there were cubs there so I was protecting the pride's cubs as they were kind enough to let me stay without judgement. So, I started fighting this other demon and they had the abilities of "grabs you and throws you across the den into the wall shattering your skull" only much less literate than that. Now, I played along for the time being but it was just too much really. (No, my skull wasn't shattered, I was just knocked unconscious from the blow for a while) .