I like the background to the Dominator Cohort: long before the Heresy, these warriors were the chosen few of the Iron Warriors and sworn to protect and serve Perturabo in battle. However, the Iron Circle automata came along in the Battle of Phall and effectively displaced them. Their hatred toward the automata subsequently increased and they became the resentful elite of the Iron Warriors machinery, despite their own failings.
This is a unit of thunder hammer terminators that hate cybernetica and can replace the Iron Circle as their Primarch's bodyguard (in which case they drop their hatred rule and don't occupy a force organization slot, as well as gain feel no pain). They get a limited choice of upgrades (but a really nice multi melta option!), and a nice bonus pip in weapon skill. Implacable Advance and Stubborn round out their rules.
Honestly, they appear on paper to be a bit "bland". On the battlefield this is probably true as well. I suspect the uses for them are against Mechanicum opponents, and can be used well in combination with the Primarch too. But their utility is very situational and they're not that much better than a baseline squad that can otherwise be built from the regular elites choice.
Don't forget the land raider Phobos to taste for the builds below...
5 Dominators, 1 with a multi-melta (300 points).
My baseline unit.
5 Dominators, 1 with a multi-melta, 4 with combi-meltas (328 points).
Melt those robots out of your sight.
10 Dominators, 2 multi-meltas, 8 combi-meltas, 5 chain fists (581 points).
Probably a good version for large battles actually.