Jaraxxus is a good card (MUCH better than Mistcaller), but he does carry a huge tempo loss, but great way to easily get it back too.
Not sure how many of you actually played the Erdard Lord, but he doesn't really bring anything to board when you play it. Yes, you can attack, and thats it. You need to have a clear board or be taunted up to not get steamrolled. Yes, 6/6 add up, and you win most of the time you get 1 or two out, but you played nothing (w/o Emperor shenenings) the turn you went Oblivion, so unless you manage to recover the turn after (usually means re-clear the board and/or taunt up), you can go down easily. Ok, you are less likely to die to some less bursty classes, but even against Pally I find myself in this weird spot where I just cant aford to lose a turn Jaraxusin'. Even if he just goes Boom that is enough presence to threaten possible lethal if you cant deal with it.
@Awaclus - Jaraxxus won't let you go to fatigue, most of the time. You need to deal with 6/6 each turn and than find a window to burst him (Ysera Awekens/Grommash is the most effective). Edit: at least I dont. Vs warrior you are the beatdown and need to wear him down before fatigue.