I think they are a very cool army. Nothing quite like fire duradin. It's basically an entire Slayers army. If you really want to be inspired, I recommend listening to the new Gotrex Audiodrama Realmslayer. There are some fantastic fyreslayers stuff in there.
The real strength of the army is a few 30 man mobs of Volkite Bezerkers with slingshields. With the shields, they're resilient (if they didn't charge), and with 20+ models in their unit, they became even more durable.
Then, if they are within 8" of a battlesmith, they can re-roll save rolls too. So, those two things combined makes a fairly durable unit.
The heroes are all cool, they have a cool monster, and the Auric Runesmiter can get around their low movement by allowing a unit to tunnel!