I love the scheme of the new race; it's a great looking model, and conceptually the racial tree fits well within the whole idea of it. A few tweaks do need to be made to it though. The Obscuring darkness cast time, coupled with the stationary position of the ability make it unattractive to use most of the time. I would ask the devs to consider making the cloud something that is placed on the caster, and follows the caster for the duration, or to make the cast time instant cast instead. The other issue I have observed, and have heard multiple complaints about from others, is the shared cooldown of the scorch SLA with class based Scorch SLA from the fire savant tree. If you fixed these 2 issues Tiefling would be on par with the Aasimar race as far as utility and entertainment for game play go.
I have looked forward to seeing Tiefling as a playable race for a long time, I would ask the devs to please consider my suggestions! Thanks!