My CSM army is entirely 1K Sons themed. I would say Sons are a very noncompetitive legion, though I've had some good luck with them at higher points levels (1750+), which allows you to take 3-4 10 man squads, plus other useful stuff like a heldrake and cultists. Ahriman is hit or miss, but his warlord trait is decent. They really need a legion supplement. Badly.