It's interesting - you're chopping 2 mana and 7/1 in stats off of Onyxia to get the full board every turn - but it only really gets good if you get a second activation of the ability. What are the odds that this minion would stick around a turn cycle, and your opponent would play at least 6 health of minions for you to kamikaze your tentacles into given that they couldn't remove the main body? That said, your opponent is asking for trouble if they can't get rid of it *and* don't play anything to try and stop you getting value off the ability.
I actually think the card may be deceptively good though - specifically in classes that can buff the attack of a whole board (druid, shaman, quest hunter) - give this a single slot alongside Unseal/Bloodlust/Roar, then basically ask the question of the opponent - "can you stop most of these tokens, or are you going to take a whole heap of damage from my buff card?". If you've built your whole deck around tokens then running Bloodlust or Savage Roar won't be a bad move deck design wise, and this card basically just asks a very serious question of your opponent. Is it the cheapest full board card we have that doesn't mirror it on your opponent's side? (There's that Imp spawning guy, but yeah, he gives imps to your opponent too so they'll likely just trade them off)
The main problem with Shu'ma is that it's an end of turn effect, which really throws a wrench in it's viability. Unlike most other board fillers that fills your board with a Battlecry effect, this card will only do so once your turn has ended, so forget any kind of large-scale board buffs because your board of 1/1s are likely going to be swiftly dealt with before they have a chance to receive any.
Unfortunately, I voted Bad on this card and I don't think it'll see any actual constructed play. In Arena, I think this is a pretty okay card.
Good versus Mass Hysteria, poor against Brawl.. I'd compare this card to The Forests Aid. It's weaker because less flexible and stats but can be a little harder for opponent to clean up. If Token Druid gets support this should be included as a third Forest aid you can play one turn sooner.
In Wild can be used turn 9-10 with Crypt lord for a 1/12 taunt that will shut down other tempo decks.
I don't comment on how good or bad a card is until the last card is revealed. Who knows. We may see a 1 mana battlecry "Trigger your minions end of turn effects" and a 1 mana hunter spell "Give your 1/1 minions charge".
This card is very underrated, exellent in a token style deck. Its hard to remove Shu'ma and the tokens at the same time, and if they can only remove the 1/1s you get to the same board again for free, still pressuring the opponent with a savage roar/bloodlust/soul of the forrest.