I hardly think it is broken at any cost; Magus of the Jar is a unilateral draw 7 card (your opponent doesn’t get to keep their 7, though they could potentially use any instants) and sees absolutely no play whatsoever. Generally, draw 7 spells are used in the process of going off, so whether or not the effect is repeatable is irrelevant most of the time because you’re going to have a fresh hand and probably win. A deck that uses draw 7 for some purpose other than comboing out is not what needs to be worried about, and I think it’d be a lot of fun.
To be honest, I think it probably is not even playable. Dragon Mage costs 1 more. Sure, he can be chumped, but how often are 5/5 fliers chumped? Not very. Most decks don’t have cards that even –can- chump it, and even if they did, you’re playing red, so probably have the tools to deal with it. Yet Dragon Mage saw practically no play. Why? Well, it turns out that giving your opponent 7 cards gives him the tools to kill your dude, and if you have a 5/5 flier out beating them down you’re probably winning. So he’s actually a tension card, and as well all know, tension cards tend to suck.
This guy does away with the whole ability to kill people directly for simply drawing (and being -2/-2) and costing 1 less mana. Obviously the best use of him is some manner of twiddle-type spell, tapping and untapping him and drawing two dozen cards in a turn. The problem is that twiddle spells are somewhat limited in versatility, and the bilateral draw means you have to have some way of breaking deck parity to deck out your opponent – and in the meanwhile if they have any instant speed answers in their deck they will see them and be able to nuke your draw 7 critter. And if you can pull off this deck, this is a turn 4 kill at best, and probably more like a turn 5 kill given your need for red mana and the lack of red twiddle effects. And a turn 4 or 5 kill dependent on a 3/3 flier that costs 6 mana and gives your opponent the opportunity to kill him by letting them see dozens of cards doesn’t sound like it is likely to be a problem, or even good.
You could try a broken jar style deck, with Megrim (and/or underworld dreams), but those are off in a third color and have heavy black commitments. Sure, maybe in Legacy it might be vaugely interesting, but there are better ways to win in that format that don't involve running janky spells like Megrim.
Summoning sickness is, I think, what makes these creatures weak. Magus of the Jar sees no play at all because he’s too vulnerable; the type of deck these guys go into, they’re one of the few targets so they will get nuked. Both can easily be cast turn 3 via accel, and come online turn 4. Yet I doubt either would be just because they aren’t worth it and there are better things to ramp up to.
I think his ability is very flashy, but looking at Dragon Mage actually makes me feel better about him.
Your card begs to be "cheated" into play, generally has no business attacking (since you want to be able to use that juicy ability,) and there's no simple way to prevent you from using the ability.
Cheating him into play is cute, but reanimation spells cost 4 mana these days, and you have to get him into your graveyard somehow. Akroma is in standard and is a better reanimation target. He is cute in a reanimator deck (as he can help set up your next reanimation spell), but again, I think generally you’d rather have an unsolvable SSS or Akroma style fatty than a 3/3 that doesn’t even have haste and won’t win the game on his own.
