Should they be called "Rubric Marines" since the other three cult units don't get called by their old legion name? I don't know, but it's up there with the top questions I have about the new codex. Then again, I suppose the commanding sorcerer is not strictly a "Rubricae". But I digress.
Thousand Sons in the new Chaos Space Marine codex are still shooty, and their AP3 bolters are going to hurt enemy marines. They've lost the "sorcerer commands" style rule in favour of simply everything being slow and purposeful. This isn't so bad, but I was fond of the old style rules simply for fluff reasons. And they all get Veterans of the Long War for free in their price.
But, they are the priciest of all the four cult marines. Most of this cost is tied up with having a level 1 psyker in the unit, coupled with the AP3 bolters. As they're a Tzeentch psyker, they have to select their power from the Tzeentch powers. Given that we're setting this unit up as a shooty unit for the main part, that means that doombolt and Tzeentch's firestorm are the optimal result. Unless we're taking a rhino transport: the odd well placed boon of mutation can be superlative. (NB: breath is a Warp Charge =2 power, so we can't have it!).
In terms of the upgrades, the aspiring sorcerer can take both a gift of mutation and melta bombs (but nothing from the ranged or melee table!). I'm a bit "meh" about the boon of mutation: I don't see these guys getting in to combat frequently, but if they do, the force weapon could prove very good when teamed up with a gift of mutation. Melta bombs again suggest engagement with enemies, so probably not worth taking since we're going to concentrate on shooting.
An Icon of Flame provides the unit with the soul blaze ability. If it were slightly cheaper, or their bolters weren't AP3, it might be worthwhile. But it's just on the edge of not quite being worth it in my opinion.
Here's a couple of suggested set-ups:
5 Thousand Sons, Imperial Bastion fortification with quad-gun (275 points)
A good armour save (3+), a good invulnerable save (4+) and a cover save. Use the sorcerer to mount the quad gun and shoot away. Fire those other boltguns at anything in range (or use the sorcerers power -- you selected firestorm, right?). Go inside the building as well if you like! Replace the Bastion with an Aegis defence line to taste.
10 Thousand Sons, Chaos Rhino with Destroyer Blades (315 points)
Ride up to the target marine squad, get out and rapid fire them to oblivion. Should the rhino survive a little while, use to tank shock enemy units on an objective. Did you roll Bolt of Tzeentch as the psychic power? If so, use on enemy vehicles from the back of the rhino along the way for extra impact.