Hearthpwn

Everytime I see someone use him they BOTH fill their board AND rank up really fast, and they completely dominate. How is that possible with his hero power? Sure, his minions are cheaper, but rolling costs more, and tier upgrades cost the same IIRC?

They have easy time in the early game, so they can upgrade the Tavern, without losing fights. And they don't really care about rolling, because in the early game they win on sheer numbers alone.

Also, if you play him correctly, you just dont roll until late. Pretty much anything you pick with him is good even if lacking synergy because by the time you get two good minions, he can have at least 3 craps that win by sheer pressure.

Also, if tokens show up on Millhouse first turn, he really shines because then can acquire even more minions earlier.

Finally, he is good at going for triples because of reduced cost, and also at going for Zerus for the same reason.

If your tavern lays out with nice minions without having to refresh, you'll do very well with Millhouse, but if you have to roll even a few times... well, ugh.  I don't really get how people also manage to level him up so fast, since his levels cost more than everyone else's, at least not while also getting so many minions, but maybe they get luckier and don't have to refresh :p

Like others said, you don't re-roll. You do it with the minions you get presented. 
You own the board early game and use token to reduce the extra costs you have to spend for upgrading.

From my experience you don't play this hero to get first. You play it to get at least fourth, as the endgame is a lot harder than any other hero.

Forgot that i was on the battlegrounds section for a moment and i was kinda confused