It casts lots of things. FoW, ravenous trap, gush.
It gives you an improved scry 2 (which in grindy blue matches is better than a 2/1 body), feeds delve and doesn't target (which is great against non-definitive grave hate).
I don't know if it's better than snapcaster in vintage. I'm trying a 3 snap/1 JVP/1 briefing splash in URw mentor, and so far it has proved to be very useful flashing back a ravenous trap and digging deeper to good cards. The problem in this specific deck is the lack of finishers (3 snap/1mentor, besides the 2 JTMS).
So an interaction I overlooked until it came up in play testing.
Mission briefing says "if the spell is put into graveyard THIS TURN, exile it instead."
When you cast a spell with rebound it never hits the graveyard that turn, so you can chain mission briefings as repeatable ways to use time walk and ancestral recall.
Edit: would need a way to get rebound from graveyard*
I've started testing this card in Oath. So far it has been awesome! Using two Mission Briefings allowed me to take strong new lines of play. Oath has experienced a downturn over the last four months or so because of various decks and the lines that were available to them. I think this card gives the deck the options that it needs handle and take advantage of situations that it previously could not.
@fsecco Sure, you can choose to play more counter spells in your Oath deck such as Pyroblast and Misdirection. You can also use Merchant Scroll, Demonic Tutor, and Night's Whisper. These cards allow you to defend against your opponents sorcery of instant speed blue spells by waiting with Mission Briefing to cast a Force of Will, Flusterstorm, Pyroblast, or whatever counter spell you might have. The tutors allow you to search up an instant speed cantrip, or whatever else you might need, and threaten to cast it again at instant speed.
I typically did not play Gitaxian Probe, but the options stated above have incentivized me to do so. These play patterns give you more options to combat Paradoxical Outcome without requiring an artifact that is potentially counterproductive to Oath's strategy. It is basically the Snapcaster Mage effect that the deck needed. The ability to cast more counter spells like Pyroblast and Flusterstorm is especially good against Teferi Control decks. The new creature card Emissary of Grudges is also a really good sideboard option against this deck.
A specific example of this card producing value was the following:
I was on Ritual Storm playing against Oath.
I cast a big must counter threat and my opponent has two options. His hand was force, Mission briefing, blue card and he had another force in his yard.
Option 1: play Force from hand and pitch blue card (or mission briefing)
Option 2: play Mission briefing to cast force from the yard and pitch his blue card, and still have a force in hand. Normally that doesn't sound so good, but the briefings surveil 2 made sure he would draw another blue card next turn to have his second force online.
It's not an insane play, and iam not sure how good the card actually is, but it definitely opens up new plays for a deck not really able to cast snaps, and produces marginal value. It is definitely playable, that's for sure, is it really good? No idea, needs more testing.