*looks up "Millhouse Manastorm" on the Hearthstone gamepedia*
Millhouse Manastorm is meant to give you an early advantage in any game. If you play first he can be a turn 2 4/4, and if you go second he can be a turn 1 4/4. Either way he is extremely good for his cost early game. The only downside is that spells cost 0 for your opponent next turn, so you had better be careful what type of deck you play him against. This can have unforeseen and devastating consequences, such as early-game Pyroblast, Mind Control, or Sprint.
For this reason Millhouse Manastorm is not generally recommended beyond the early game, unless your opponent has only 1 or 2 cards in hand. He is most perilous of all against mages, who may be able to defeat your hero outright using the damage spells they have in hand. Certain mage decks such as Freeze Mage intentionally hoard damage spells over the game, making a Millhouse an invitation to unload 20 or more damage in a single turn.
If played under the right conditions, Millhouse can be a cheap 4/4, and in the first few turns can create a significant increase in tempo. In the later game his low cost tends to be fairly insignificant, while the potential for backfiring increases as the opponent's hand grows.
Yikes! That's quite a downside he's got there! 