While I can't speak to Grav, I have a Dark/Dark Dom at level 46, and he is a blast to play. He solos easily and does decent damage, but that depends more on your secondary rather than the primary. Dark Assault is very DoT heavy, so you end up with lots and lots of little numbers melting them away over time, (especially with my build that uses Enflame cast on my ghosts on top of that, DoTs everywhere). If you want something more bursty you may want to choose a different secondary. But Dark Assault does pair well with Dark Control to get stacking tohit debuffs. It's no Dark Miasma or Rad, but if you have a bit of defense happening from maneuvers, or stealth or whatever, it adds up to some pretty solid extra damage mitigation. But I think Psi or Fire would also pair well, depending if you want a more ranged or melee playstyle. I favor ranged, personally.
But it's very solid AoE control, once you get Fearsome Stare, and I love sets with a confuse, it's so handy to confuse mobs with buffs or heals, or if Domination isn't up, and the group is not aggroed, i like to confuse bosses before using holds on them, so i can enter the fight with the boss not attacking me. Though a Shadow Field then immediately holding the boss also works. My usual is to open with a Possession since it doesn't aggro, then Fearsome Stare, Living Shadows, Haunt, hold and /or confuse some guys, and AoE till they're dead, more or less. Shadow Field if there are adds, the map means a cone won't hit them right, or they are resistant to Fear (damn Nemesis), or i just want some extra tohit debuff, or i am trying to lock down multiple bosses or a tough opponent like an EB/AV.
On teams you will likely end up spamming Fearsome Stare and your attacks most of the time, honestly, maybe Living Shadows if managing knockback is a concern. Most other controls will be wasted as things tend to die too fast. But AVs will benefit from more of your skills, you should be able to easily lock down an AV with Shadow Field and your single target hold, triangles permitting.
You'll also greatly benefit from a lot of +rech even if you don't opt to go all the way for Perma Dom, Since Shadow Field, Haunt, and to a lesser extent Fearsome Stare have hefty recharge times, so definitely fit Hasten in there.
I took some video of me playing the character at level 36, he's a bit more beastly now. I may take more once i hit 47 and have World of Confusion.