These robots have long featured at the forefront of the Mechanicum's armies. So much so, that some in the Imperium saw them as a sleeping threat should Mars ever decide to break away from the deal struck with the Emperor. The Heresy quashed that in short order though.
There are some amazing stats associated with this unit, not least the T=7 characteristic. That's going to take some beating. And recall: these robots are troops as well, hence they can be taken lots of times in an army (but they don't score as they're classed as support units). Its going to take something special from the likes of space marine legions to be able to reliably combat them, and if not: specialist units that should probably be firing at other targets. They are therefore a great threat on the battlefield in my opinion. Add in to this that fact that they have 4 wounds each, and you can see what a headache these monstrous creatures actually are.
On top of this, they can be configured in a number of ways. Melee can be achieved with power blades, and a ranged tank killer can be made with some dark fire cannons.
The significant weakness here are that these guys are support squads. Arguably the low WS may be an issue, but in reality, I think they're going to do just fine in close combat due to their W, S and T characteristics. Of course, the cortex cybernetics is a draw back, so you'll need a cortex controller somewhere on the field of play as well.
Just a couple of builds here to whet one's appetite. Others are certainly possible.
5 Castellax, three with two battle-automata power blades, frag grenades (480 points)
I've kept a couple of the castellax here with their shock chargers, just for the concussive special rule that they have. There's very little not to like here - apart from getting them in to combat to do the damage. Equip with flamers to taste, and possibly siege wreckers too depending on the mission / likely enemy.
3 Castellax, all with Dark Fire Cannons, Enhanced Targeting Arrays (360 points)
This is the shooting maniple of death! With a penalty to cover saves, a higher BS, and high strength weapon that pumps out 6 shots at S7 every turn, this is a unit to fear.
1 Castellax, Multi-Melta, Frag Grenades, 2 Flamers, Paragon of Metal (135 points)
A very reasonable price to pay for a dreadnought-like robot togo stomping around the battlefield looking for targets to take out. A threat. Or a distraction, as required. Note that programmed behaviour is not such an issue for this build, he may not need a cortex controller nearby. I'd like to hear from anyone who's done without a controller to hear how it turns out :)