With this card, we now have 4 Protection cards that do ALMOST the same thing with finicky differences, the other three being True Grit, Jump Flip and Hard to Ignore.

True Grit is probably the worse of the quartet. For 2 ER, you remove at most 2 threat (well, 3 if you're playing Phoenix, Spectrum in Photon form, or max level Ironheart), which is a mediocre ratio. It also hasn't any particular synergies with other Protection cards, since it's a THWART Event, not a DEFENSE. The silver lining is that you're not limited to removing threat from the main scheme, but it's not saving the card from utter mediocrity.

Jump Flip and Hard to Ignore are arguably on par with each other, each suited to slightly different scenarios but also capable of synergizing well with each other. Jump Flip is more flexible as it's not dependent on defending using other means than itself, but it has an annoying energy resource requirement (which nevertheless works well with Nerves of Steel), while Hard to Ignore has a better threat removal / ER ratio in the long run but has a no-damage requirement that makes it better suited for high-defense heroes who like to defend a lot, in conjunction with Armored Vest, Expert Defense, Desperate Defense, Never Back Down, or Riposte. However, the big drawback here is that both only remove threat from the MAIN scheme, which sometimes can be very limiting.

Personally, I think that Not Today! is the best of the four. It's similar to Hard to Ignore in terms of what it does and how (and in fact it benefits from the same card synergies), but it's not limited to removing threat from the main scheme, intrinsically helps you reach the no-damage requirement, and it's a DEFENSE Event which benefits from cards such as Flow Like Water, Defensive Energy and Nerves of Steel. It synergizes well with its three brothers, but I would never include all four in the same deck (certainly not True Grit, and I would choose Jump Flip over Hard to Ignore only if my deck reliably provides a lot of energy resources and/or my hero isn't very good at avoiding damage entirely). Combine with Bait and Switch if you're playing Solo against heavy-scheming villains, or in multiplayer to be hailed as the definitive support hero (especially if there are no heroes with the Justice aspects).

I am surprised that this card has no reviews yet!

For me, it's an auto-include in any Aggression Thor deck. Thor becomes Aerial thanks to Mjolnir and loves to use it to take a Mean Swing or a Skilled Strike against the bad guys, so this card is basically a much better Jarnbjorn with improved resource efficiency (damage/ER ratio of 2, on par with Hand Cannon), AND you can play it for free with Team-Building Exercise since it shares the Aerial trait with you (EDIT: most likely you can't do this, see discussion below).

If you happen to have Mean Swing, Skilled Strike and Pitchback in your hand, you can turn a basic attack into a 12-damage wallop for only 4 ER, which is amazing value! 3 copies of each of those cards are enough to defeat both stages of most villains in the game when playing Solo.

Of course it's good for other Aerial heroes who attack a lot, like Nova (it's in his pre-built deck for a good reason) and Spectrum, and obviously super good for Aggression Angel who draws a card after playing it, slightly less so for Aggression Ironheart who doesn't attack as often. Grab it together with Dive Bomb and you'll have a solid backbone for any Aggression deck based on those heroes.

I don't like this card.

This is the S.H.I.E.L.D. team card, but its not really a team card... every single other team card buffs, or otherwise deals with allies:

Mighty Avengers makes allies stronger.
Guardians of the Galaxy gives card draw when you upgrade allies.
Uncanny X-Men gives allies more health and makes them cheaper.
Uncanny X-Force makes allies better at thwarting and gives them longevity.
The upcoming Flight Squadron team card for aerial gives ally readies when playing aerial events.

Everything makes allies better, and you're encouraged to play allies to make use of the team card.

Agents of S.H.I.E.L.D. does none of this. It's an encounter card mulligan. While good, it has nothing to do with allies. Furthermore, it discourages you from playing allies as there is no buff to be found for them, and having an ally that doesn't have the right trait will completely disable the card. Instead of giving you something by playing a tribal deck, this card takes it away by not playing one. In fact, you can play this card and use it while having 0 allies on the field, not very team-player of you.

The main reason I don't like this card is that it's taking the space of the S.H.I.E.L.D. team card and therefore we wont get another one. This card should've been something else, perhaps another location support or a title, or anything else other than the singular S.H.I.E.L.D. team card. The other team cards are awesome and lead to interesting strategies, this one has none of that.

But for the power of the card as-is, encounter mulligan is strong and welcome in leadership, I just wish it wasn't the team card.

I'd argue this is the most important card in Maria's kit. I'll often run Superpower Training just to help make sure I can get this down on the board. It's mostly the fuel that keeps her Helicarrier engine running, but I'll sometimes throw an extra counter on Med Team to help keep Nick Fury topped up (which winds up being another 2 counters) or Support Staff for another wild. Plus you can use it to basically stock up on counters since Maria can move a counter from one support to another each turn. Just overall an invaluable upgrade.

Extremely good card for Protection Thor (who becomes Aerial thanks to Mjolnir), or Spectrum in Gamma form, granting 2 threat removal plus at least 3 damage for 3 ER, an astonishingly good ratio (compare with Hit and Run which only grants 2 damage and 2 threat removal for 4 ER).

Obviously good for Angel as well, who draws a card when playing it, and for Nova, who relies on basic powers to generate resources from his Supernova Helmet, less so for Ironheart who, even if Aerial from the start, has lower basic stats and no particular need of readying herself often.

Many other characters can benefit from this card (Captain Marvel, Iron Man, Wasp, Star-Lord, Vision, War Machine, Valkyrie, Phoenix, Storm, Rogue, Magneto), although none of them is Aerial right from the start like the ones I mentioned above and thus have to rely on cards that make them become so. Personally, I wouldn't include Aerial cards in these characters' decks if I have to rely to a lucky draw, except maybe Iron Man and Magneto who have the means to tutor the cards that make them Aerial.

However, special mentions go to Doctor Strange, who gains Aerial from his Cloak of Levitation and loves to ready himself so he can cast more invocations, and Iceman who gets Aerial with his Ice Slide and loves to use his basic powers to attach more Frostbite to his enemies. If you play these characters with the Protection aspect, Ever Vigilant may be worth including.