Well, it's about time! A useful blocker for mill decks. Strong enough to make weenies think twice, and tough enough to make a staunch defender. As for the ability: it is exactly what was needed. It's not so powerful to make blue overpowered and plays nicely with the other colours.
A staple blue card if ever I saw one. It flies and has higher toughness, it's ability is devensive and it adheres to the blue philosophy of messing with the enemys' library. Its ability would usually resign it to Mill decks as a mainstay, but I would consider using it in most of my blue decks that wanted to play mindgames with the opposition.
Strategies and game play:
Having only a single blue in the mana cost means it works well in multicolour decks (as Ravnica practically requires)
Combos with other cards:
An Entangler and some way to regenerate would make this a defensive powerhouse.
Ways to counteract it:
Seeing as the opponent is hardly likely to attack with it, you'll be wanting to pacify it or Destroy it. Try not to damage it unless you're absolutely sure you can kill it in a single blow.
Tough flying creature that acts as a deterrent to would be attackers
5 mana means it wont hit play early. Personally, I'd prefer a 1/4 non flier with the same ability for (u)(3)