Does the player get to see what critical effect he'd be receiving before he decides whether he's going to cancel it with the armor's ability?

Well, the game doesn't have nearly as tight timing rules as something like Magic: The Gathering, so it's impossible to say for certain what the intent was, unless someone on here goes and asks somebody at FFG.

But I can see two arguments pointing in two different directions:

• The cost depends on the severity value of the crit, therefore you need to know what the crit value is and you can only know that if the card is revealed. Therefore, you should get to see the card first.
• The gromril armor is really good. Really good. There's a thread around here somewhere about just how hard it is to do any damage to an Ironbreaker. Letting them see the crit before making the decision just amplifies the thing that the career is already better at than all other careers. They don't need that boost.

I'm inclined to say the first point more closely matches the designers intent, and the second point more closely matches the reality of the game mechanics. Which means picking between them is essentially a matter of taste.

I'd look at the dynamics of your group. Know your group, and you'll be able to rule the best way for your game.

If all the PCs are combat-characters, then I'd rule that the Ironbreaker gets to look at the crit first, as the extra bit of power will help distinguish that character from the others. The gap between PCs is small enough that giving the dwarf a shot in the arm will be a benefit to everyone.

If the party includes characters that are definitely not fighters, and never will be, then I'd be inclined to say they have to use the power before they find out the nature of the crit. It reigns the power in a bit. The reason you'd want to do that is to take a bit of the sting out of the gap between PC capabilities. If you've got an Ironbreaker and a Scribe, foes that are the least bit challenging for the Ironbreaker will one-hit-kill the Scribe. While that certainly fits the gritty feel of Warhammer, it can be really hard to GM that well over a long time. (The abstract movement rules let you the GM get around this from time to time, but in a long campaign you might not be able to make drama and mechanics work in synch 100% of the time.)

The most pertinent question might not be "what is the cards intent?" Instead it might be "what will play best for our group?" If there's a couple squishy social PCs, will they be happy trading off spotlight time with the tank based on whether or not there's a fight going on? Or will they decide that to stay competitive they need to take soldier as their second career and pile on the armor, leaving their original character concept in the dirt? Will the Ironbreaker in the group be willing to trade (or allow them) spotlight time, or is he (or she) the type that likely to get bored during social scenes and pick fights with the NPCs? Will the Ironbreaker revel in being the unkillable main target of the enemies, or will that player get upset that all the best attacks keep hitting him? Make the decision that leads to the most fun for your group.

In the absence of any other factors, I'd rule in the favor of the Ironbreaker, and let them look at the crit before deciding to use that power. There is, after all, a good chance that the Gromril Armor was the reason they chose that career. You don't want to create a situation where they made a significant character investment (choosing that career) expecting a certain benefit, and now feel cheated (or just let down) because the card doesn't work the way they thought it did.

Well, for starters, it's perfectly possible to require activation of the power before looking at the card. the player says whether or not he's using it, then flips the card and pays the cost.

I agree that it would be best to rule based on the group, but we're starting a game today and doing character creation. So the main reason I'm asking is in case someone draws that as an option for their first career, he's probably going to want to know how it works, and I'm going to want to rule on it. At the moment I'm leaning toward requiring the player to use it without seeing the crit since I feel like the ironbreaker is plenty badass even with the ability "limited" in that way. But I think I'll submit it as a rules question.

You can always just say the severity rating without revealing the crit effect itself as a "compromise" rule.

Well the official answer from Jay is: "The player may see the critical wound card before deciding whether or not he wishes to convert the critical wound into a normal wound by using the Gromril Armour's special ability."