This is ancient watcher with +1 cost, attack, and health, and a downside that's not as bad. Seems like it might see play in silence priest, or tempo mage. You can follow this up with the arcane anomaly the next turn, which is a good start to a curve.
Above all Mana Reservoir is a great spell damage card with staying power, which makes Arcane Watcher and Spellbook Binder all the more viable. Because even if you don't have spell damage minions on the board they have to deal with Arcane Watcher because you could play a spell damage minion at any time.