Pretty fun! This is a solid concept, but there's some balancing to work out and there's still some broken stuff (Extra Hand Does Not Work, sadge). Also I've played a bunch of runs and still can't beat it (even with the power of Chain) :(
I do have to note that in certain circumstances, if you run out of discards, you just can't play any dominoes. This is really sucky and can easily kill a run -- it would be great if you changed something regarding the end-of-turn mechanic on dominoes.