I've ran two for a while and I've had a good amount of success with it. Its 180 HP keeps it away from the dreaded magic numbers of Darkrai, it can be searched out by Heavy Ball(also giving you another out to a Blastoise if necessary), and its weakness isn't a huge issue. The other dragon type that is really popular is Rayquaza EX, which OHKO's Black Kyurem EX with 3 lightning energy regardless of weakness anyway.
Getting this card set up is the matter, and that is usually due to having trouble finding the lightning energy. Having a lack of water energy is rarely an issue from my experience. To me, decks that include Black Kyurem are simply tight on space already; having to tech in cards against Garbodor and Klinklang on top of that sorely hurts its consistency.
I give this card an 8/10 for right now. I really like this card, and I don't feel the problem is with this card itself. Maybe when Plasma Freeze and Super Energy Retrieval comes out, this card will really shine.