They're definitely not the easiest army to field. What makes them a challenge is that they're most effective within flamer/bolter rapid-fire range. They shine at that 7" mark. However, that means that you typically get only one volley off before being assaulted.
The trick is that that one volley can be devastating enough to win a game. Flamers, Meltas, and rapid-fire Bolters will make short work of practically any unit.
So harass the opponent until you get there, gut their force with your mass of flamers, and mop up the remnants in assault.
Besides, you get to call Space Marine players "vile mutant heretics." How can you beat that?
Archon Xanath