The Domitar is an upgraded version of what the book calls the "ancient" conquerer design of robot (which I thought was a nice nod to the Rogue Trader era 40k!) that was brought out just prior to the Heresy happening. These robots are a really nice addition to the range as they fill in the elites spot very nicely and are able to take on marines, terminators, tanks, buildings, and most other things in between. They're a close combat beast, but one that needs some placement thought and management.
I see two primary strengths here. The first one is the obvious close combat role with a pair of graviton hammers that ensures high strength and low AP, coupled with forcing disordered charges against them. And did we mention d3 hammer of wraths -- that's great!
The second is that they provide some well-needed anti-air cover in the form of flak missiles. Clearly this is might be needed in some builds more than others.
For the points, if these guys charge the wrong unit, they're going to regret it. They will also regret tar-pitting units. Hence they need to have some well-thought through targets to charge in to and handle.
Domitar, frag grenades, paragon of metal (215 points)
Pricey, but a really nice use of paragon of metal in the right situation.
4 Domitars, frag grenades, flak missiles (740 points)
Wow -- expensive for what you get, but capable of taking almost anything on in the game. Placement is going to be an issue though.