For anyone who has seen one of the Warhammer Mark of Chaos intro movies from the computer game, Warrior Priests are lovely for their imagery and background.
Now as yet I haven't had a lot of success with them, but here's why:
I always forget to use their Prayer abilities. Or the Hatred. I ALWAYS forget that. In fact, the biggest challenge with Warhammer is remembering all the little detail like that when the heat of battle takes over.
With the new magic rules meaning Bound Spells are no longer "free," having Warrior Priests and Wizards in the same army can eat into one another's Power Dice pool so don't go too heavy with both or you'll end up with nothing.
Because the Bound Spell prayer abilities of Warrior Priests are so easy to cast and they don't have a magic level to add to the attempt, they tend to be easy to dispel too so don't rely on getting a lot of these off. Several of them Remain in Play. The key here is to lead up your Priests with these effects as they advace so that they're in position to use them when their moment comes.