On a second look this is far narrower in application than I'd expected. Landfall and enchantress was are excluded, of course, but so are cast triggers. This eliminates many or most of the repeatable triggers. Outside actual "When ~ ETB" effects how useful is this? How many decks will have enough density of ETB triggers to justify a slot?

I'd argue most above average lists will have enough ETB creatures to get a huge amount of value out of it, but it isn't a auto include by any means.

Blink, bounce and recursion are just popular strategies, so it can fit into a fair few lists pretty cleanly.

It will be one of those classic cards that everyone will say "Blah blah, doesn't do anything or win the games on its own, to fragile, dies to removal etc. etc.", but should probably see play anyway, just because it's a fun card that has counterplay and if it does stick around, it generates absurd value. The kind of card that makes games memorable.

In my opinion, This actually has a wide variety of cards it synergizes with like Purphorous, God of the Forge and Blood Seeker. I think to some extant it can cause Extort to double trigger as well as the Rally Mechanic from numerous allies, and prowess. It might even work with parrallel lives and Doubing season to some extent.

I'm not so sure about Eldrazi ETB token spawn effects though.

Extort's a cast trigger. Parallel lives and doubling season trigger off the effect, not ETB.

I can see decks that could use it, but you'd need probably 10+ cards to make it worthwhile, no?

So if I had Parrallel Lives/Doubling Season in play along with Panharmonicon, I play a Brood Monitor or Scion Summoner, Would I get 3x or 4x times the scions?

Twelve scions off brood monitor, since you'd get two triggers of 2x3 scions each. Four off scion summoner.

Oooo! i just thought of the Ultimate combination, Primal Vigor, Parallel Lives, and Doubling Season combined with Panharmonicon then play a token deck, or in my case Eldrazi, the Scions will swarm!

Well, I think it's probably an auto-include in blink decks for starters. Then there are decks whose general has ETB trigger itself (Zegana comes to mind) or triggers off other things coming in (Newzuri), or makes creatures (Kiki, Ghave, Grenzo 1.0).

I'm not actually sure how good this generically. It really feels like a downgrade from Conjuror's Closet a lot of the time.

I feel like a lot of the EtB triggers that see play don't get a huge benefit from being twice as big all at once, but do get more utility by happening again on another turn. Like Craterhoof and Prime Speaker both are big enough that doubling them is often inconsequential. The scariest thing you can do is probably something like Grey Merchant, but that's also probably going to be at least as good with Closet nearly all the time.

Of course the card will enable some big play, but the real question is how often you are going to miss a big play by not having a more pro-active card in that slot.

Hopefully no one in your playgroup runs Massacre Wurm.

Are we really having this discussion? ETB triggers have been a hugely broken aspect of the format for years. "XXXX is bad because it doesn't have an ETB trigger." has been a common phrase in every format. This doubles all the ETB triggers that are format staples as well as enables other ETB triggers to just go infinite upon themselves. Venser bounces himself and a spell/permanent. Kiki combos are no longer just generating infinite tokens, they now generate infinite mana or steal every permanent on the field. Twice as many Avenger tokens. Craterhoof is now lethal with half the board state. Primordials, Gearhulks, Titans, Gary? Double mirrorworks triggers. Double Minion Reflector/Riku creature triggers. Palinchron goes infinite by itself now, no need for a mana doubler.

I don't expect it to become a format staple but there are decks that will definitely want this and it will be super strong in those decks. You're giving up about as much as you're giving up to cast a doubling season effect and we all know how useful and/or broken those are.

I think control is where I fear seeing this the most. Man-o'-War and friends bouncing double, or themselves and target, repeat as mana allows (add Tidespout Tyrant to ensure no opponent's keep permanents out); or double hitting Fleshbag marauder effects in MBC/UB control.

I think it definitely has fun applications and I hope to play around with it (my Tribal Elemental wants it - evoke effects amongst other things). But I can also see people trying to break it, so that it winds up as "kill on site" no matter the deck slinging it.

This card is a trap. It's a four mana do nothing card that might give you some extra value. How many people actually play strionic resonator? Resonator is more versatile and costs only 2 mana and it sees basically zero play. Unless you have some sort of 30 creature blink deck is a very casual meta then maybe you can afford to play this for the lolz.

Called it.

So I think I'm going to trial Panharmonicon in Uncle Karl, who has the most reliance on ETB among my decks, and also cares more about the number of instances of lifegain than the totals.

I didn't ever really think that the card was useless, it was more a response to looking at where it would fit, and finding it far narrower in application than I'd anticipated. It went from the Frank's Red Hot of EDH (I put that **** in everything), to being just another situationally useful doubler.