Hearthpwn

You're getting it all wrong. In theory, for 10 mana, you're not just getting a minion but destroying an enemy minion as well.

So let's see, basic removal costs 5 mana. (Assassinate)  That means five mana, for removing enemy's minion and then five more for you getting that minion, which means that if the minion you get is anything better than a five cost minion it's a good trade.

TL;DR: If the minion you take control of is anything better than a 5/5 it will be a good trade.

I agree, but the reason why this card is not viable and in general not 'good' is because good cards  give you value even if they are removed with removal. E.g: Dr. Boom gives 2 Boom Bots, Ragnaros deals 8 damage, Alexstraza lets you change a hero's HP.

The cards I mentioned are just as expensive as Gruul, but they give an effect when played, unlike Gruul which might as well be a waste of mana when cleared using removal.

Deathwing is supposed to be played either as a last resort when you're losing and think, "hey maybe this card will turn the game around, or maybe it won't in either case I am losing so whatever."

Or, you play it once you know you have exhausted all your opponent's resources such as BGHs and removals and don't have any valuable cards in hand that you would mind losing.

If Deathwing's the first minion you play as you reach turn 10, then you're likely going to lose. Sorry a card requires you to actually think and judge the situation before throwing it in.