Bonemare is just a common card, but getting it in arena feels almost unfair for your opponent. It's basically a 7 mana 9/9 that adds taunt to a minion so in the topdeck wars of a 2nd part of an arena game it's almost always game-winning.
The card is also used in constructed. Before Kobolds & Catacombs it was the most dominant card in Hearthstone, around 35% of the decks containing it.
RegisKillbin suggested a nerf for this card, as you can see in the following video:
But even with a new expansion coming out last December, Bonemare hasn't lost its appeal to deck builders and it can still be found in many Legend-reaching picks.
Expansion: Knights of the Frozen Throne
Class: Neutral
Lightforge Arena Score: 162
Cost to craft: 40 dust
Current Tier 1 decks that include it: Tempo Rogue
Worth it: Definitely!