Background Material Evaluation.The Thousand Sons are painted as one of the more unusual legions within the pages of Inferno. We all know of the psychic potential (and potency), but more fundamentally, Magnus arranged the legion in very different ways than his brothers. The focus was distinctly at the squad level. These were arranged into Circles and Fellowships with other aspects (Cults, Orders) cutting across these.
Then there is the whole Flesh Change issue that the book brings to the readers attention and how at various points the legion was shunned and distrusted, nearly extinct and booming. And how lots of them belong to the Dreaming Brotherhood -- placed in to stasis until a cure was found.
My personal favourite bit of their history is when they send a mere 60 marines to bring a Knight world (planet 72-9) to compliance. They just stand in place, soaking up incoming fire with kine shields and then rip the pilots apart before telekinetically tossing the Knight's head back at their base. And thus they lost three marines; two to headache issues and one to being squished by falling debris. Magnificent!
Legion Rules Review.
I regard the Thousand Sons as potentially superbly powerful and a highly tailored force that would even make the Alpha Legion cry with jealousy.
Covenant of Sorcerers means that their warlord must be a psyker, and the one with the highest Ld score. In turn, this implies (at least until FAQ'ed) that it would be hard to impossible for most forces to take Thousand Sons as allies.
Proserpine Lore gives psykers access to every discipline apart from Malefic, and the player can purchase psychic levels and Force weapons for their HQs at a price.
Cult Arcana is the rule that sets them apart though. Each unit must select one of the cults (with compulsory troops possessing the same cult as the HQ). Psychic powers associated with the cult manifest on a 3+ rather than 4+ for warp charges. The cult grants bonuses (+1 run and sweeping advance; +1 invulnerable save; re-rolls of failed to hit rolls if stationary; adamantium will and fear immunity; or an extra hammer of wrath attach). These are very powerful and can be abused (combo-ed) to a huge degree. Naturally.
The HUGE drawback is the Signs and Portents rule. And it is a BIG one. If a perils of the warp causes wounds, they think the flesh change is back. Every unit in the force takes an immediate pinning test. Your heard that -- every single unit. Death of all the independent characters causes every unit to suffer -1 Ld for the rest of the game and may no longer sweeping advance.
Overall then, the Thousand Sons are an elite, expensive, infantry based force and filled with psychic might. Heck, they might as well just have two shooting rounding every turn thanks to the mind bullets that they're going to generate. On the other hand, they can become crippled with a couple of poor rolls. The more cautious general might like to stick to power level 1 powers for most units and characters. And/or purchase the arcane litanies.
Finally, it should be noted that Praetors can (must) purchase psyker levels and the veterans and terminator squads have the option to upgrade to be a brotherhood of sorcerers.
Arcane Litanies is almost an automatic purchase to get around a single bad perils of the warp for an independent character. Well recommended.
Aether-Fire Cannon adds soul-blaze to plasma. Just not worth it really. I get the push for soul blaze, but not on these weapons.
Asphyx Shells yield shred to bolters (ICs and terminators and veterans; or to rotor cannons for support squads). This is very nice indeed and worth taking. It makes rotor cannon squads in particular rather viable.
Osiron Dreadnoughts can also be purchased as an upgrade to regular contemptor talons. For 50 points, they can have psychic powers, adamantium will, force blade and asphyx shells. Worth taking if you have the points realistically.
Rites of War.
The first rite of War, The Axis of Dissolution, is a bit situational to say the least. Bonuses to overwatch are nice, re-rolls against falling back enemies are okay, but the best bit is automatically passing pinning and morale checks. But only if they're on top of objectives. Having to take maximum troops is a pain though.
The second is nicer: The Guard of the Crimson King. The players gets a bonus 1d6 for warp charge generation at the start of the psychic phase and the player can count the highest roll on this or one other die for harnessing powers. It gets that first power off nicely realistically. Allocating deep strike to terminators is great (of course) and causing fear when they arrive is just icing on the cake. However, Sekhmet Cabals have to be compulsory troops and Magnus may be purchased as the HQ. Fair enough.
I see the Thousand Sons as mainly infantry based, albeit with rhinos everywhere to get them to places. They are also points intensive thanks to all the upgrades and psychic powers knocking around. Hence they are elite. But they're going to get effectively two shooting phases that should be taken advantage of. They should do well against psychic Word Bearer builds very well. I don't know how they'd find Eldar though.