I think they're alright. They give you another option for mana fixers. I like Cascade, particularly if they make enough of them. Ripple never worked out as well as I wanted, this might do better.
They're great in limited, and there was an article on Starcitygames that pointed out they're also very good at getting your Knight of the White Orchids for full value, even when on the play.
Deathbringer Thoctar would have excellent synergy with Arcbound Ravager, not that anyone using Arcbound Ravager would bother to use Deathbringer Thoctar, but the option is there...