The short version of this card is turning every
aura and equipment in Magic into a combat trick.
How good is that in practice? I suspect it's
actually stronger than it sounds. Sorcery speed
was pretty much the only thing keeping cards
like Sword of Light and Shadow and Armadillo
Cloak from being ban candidates. Even things
that aren't quite as crazy as those play
entirely differently when they have flash;
consider something like Leonin Bola, which is an
annoying trick held back by the fact that you
always see it coming.
I feel like Sigarda’s Aid might be the sleeper
hit of the set, waiting to break out in either
Modern (where there’s a lot of powerful
equipment, like the Sword cycle and Batterskull)
or once we get better Aura or Equipment spells
in Standard. It's a powerful effect: give your
Auras and Equipments flash, and have the latter
pop onto a creature immediately upon entry. I
can see Modern abusing the hell out of this with
something like Batterskull.
That said, this card is as powerful as the tools
it has to play with. Standard is not a format
presently heavy on Aura spells or Equipments, so
this is apt to underwhelm. But it's cheap
mana-wise (1 mana makes it wholly feasible the
same turn you’re planning to unload your
relevant spells), which is definitely a point in
its favor.
I think this is a card to watch when we go to
Kaladesh. It's merely okay now, but it could get
all the necessary tools in the next block.