I dunno, I don't think they are any worse than Plague marines.
Plague Marines don't have to take a sorcerer, have defensive Grenades, T5 and feel no pain, can take special weapons and the champ can have a fist - something I always like to have. And if I take 7 and give them all the bells and whistles they'll still be cheaper than a unit of Thousand Sons, and are FAR more versatile.
I think BEserkers are just really nasty for a low point value. The good thing about the sons is that they can be both light and heavy infantry killers, freeing your heavies and elites for anti-vehicle.
Berzerkers in my opinion battle with PMs for the top spot. Those two cult units is where it's really at.
As I said, Thousand Sons are playable, but if you look at it from a "what's better" perspective, they simply lose out.