Alright, since I have the four characters available on the DLC's initial release...
The new turians are great, the Ghost Infiltrator can be built as a melee monster. The Havoc Soldier's new power, Havoc Strike, is effectively a combat Charge that doesn't restore shields. Both of them have a new power, Stimulant Kit, that uses grenade supply and provides both a shield boost (ranging between 2000 and 4000 dependent on your evolution choices) and a damage boost. All around, they're very nifty, and solid rare characters.
The volus adept and engineer, on the other hand, are ultra-rare characters, and I'm still trying to get to grip with them. They start with 150 health and 500 shields, combat roll, and cannot take cover. Their light melee activates a tactical cloak off cooldown, and their heavy melee erects a force field to vastly reduce their damage taken. In addition, they both possess the volus signature power, Shield Boost, which provides a hefty burst of shield restoration to the volus and all allies within range. I've chosen to go for the capacity restoration evolution path for both the volus characters, giving them and nearby allies 8000 shields for 12 seconds. Yes, 8000 shields. That's not a typo.
The volus adept also packs Stasis, and a new power called Biotic Spheres. Activating it initiates cooldown, creating three spheres. Each sphere provides a hefty cooldown bonus whilst up, and can be fired off at no cooldown for moderate damage and biotic detonation. It can be evolved for a good area of effect and a decent debuff to provide more utility against unprimed targets.
The engineer packs Proximity Mine and the new Recon Mine, an handy little power. Upon casting, it initiates cooldown. Once placed, the mine takes a while to become active. Once it does, it reveals all enemies close to it. You can also tap the button again to detonate the mine, causing heavy damage. Useful, if you can place it somewhere handy.