I've been pretty unlucky with Hearthstone boosters: 30 packs in and still no class-defining legendary, rare or epic to really build a collection around. I have, however, generated several hundred dust which I'm itching to spend on something.
So the question is: given that you don't know what's going to be in the next booster, is there an "optimal" approach to spending your dust?
A lot of sites seem to suggest that you should burn it on the most useful common cards, the ones that you need to get decks working, on the basis that they're cheap. But that seems bad advice to me because the chances of you getting what you need from boosters is quite high. After 30, I've got at least one of all the most desirable common cards, and two of many.
At the other end of the spectrum is the idea that you should save them up for the most powerful legendaries. On the one hand, the chances of you getting a real killer legendary from opening packs is very slim. But on the other, you need to accumulate a vast amount of dust and there is the awful chance of you aquiring a second and totally useless copy of the card.
At the moment I'm edging on using it to buy rares. They don't break the bank, and if you just buy one, acquiring a second through pack opening is actually pretty handy.
But this is just finger in the wind opinion. Is there a more rigorous way of looking at things?