I've fucked around with it in Legacy and my opinion is that the card is really good and strong, definitely long-term Legacy viable. I have yet to Cube with it but I think it would totally fit in my Cube. I like that it's a goodstuff card that bleeds into the pants/equipment subtheme and also spellslingers/ascendancy.Anyone played with Dreadhorde Arcanist?
to show off what I'm talking about / put my money where my fingers are, I forced a Jeskai Ascendancy draft on CubeTutor. Here's a deck that I think Arcanist really shines in:
ascendancy ft arcanist from CubeTutor.com
in this list, Arcanist at its base potential is perfectly fine flashing back removal or a cantrip (stabcaster mage), but once you get an anthem or Ascendancy trigger going, it can really go off. Let's say it's turn 5, Hero has three lands, Villain has 4. Picture this board:
pre-combat, Hero casts Lightning Helix to clear a blocker, kung-fus and loots, and then attacks with a 3/3 Menace Flameblade, 2/4 Trample Arcanist, and 3/4 Reflector Mage. Arcanist triggers during attacks, we flashback Helix for free and go face, pushing 14 damage in one combat step while also clearing blockers, looting 2, and gaining 6 life. For two mana! If we use our third land to play a bauble or something, that's 21 damage. Obviously this is more about the power of Ascendancy than Arcanist, but it's a solid illustration of just how well Arcanist supports spellslinger type archetypes, of which Ascendancy is the most satisfying I've tried.
If you run Jeskai Ascendancy (get on board already, babe) I think you should be looking at quietly supporting it with your existing goodstuff cards (this list is a great example actually). I hope I have convinced at least one person to run Arcanist!