I have about as many additions to my cube from this set as I typically do from a new set. Though, I do feel slightly disappointed because the original Theros gave us Purphoros, God of the Forge and Gray Merchant of Asphodel -- 2 cards that completely changed the identity of their colors in my cube. None of the additions from this set look like they'll have as big of an impact. I guess what I was really hoping from this set was another devotion pay-off in black, but nothing really delivered on that. A 2/1 for B with escape or could recur if your devotion to black was high enough would have been awesome. Still, Thassa looks like a nice addition for me. And although I'm not adding Heliod, at least not immediately, Heliod does look nice for cubes that want to include him.
[180 classic cube]
Seeker of the Way -> Eidolon of Obstruction
Crystal Shard -> Thassa, Deep-Dwelling
Gifted Aetherborn -> Mire Triton
Testing
Shrine of Burning Rage -> Anax, Hardened in the Forge: I may end up replacing Shrine with Krenko, Tin Street Kingpin instead.
Watching
Heliod, Sun-Crowned: I would need to shift around a few cards to include this, but I could see eventually adding it.
[180 classic cube]