What is up with Exert? Is there a deck that I'm not seeing? I'm guessing that it is meant to be mixed with untap effects so it can be used more often. Most of the bonuses granted via Exert seem underwhelming to me. What do you think?

It's mostly a limited ability I think but it could be awesome in a deck running Always Watching, though that is rare so might not fully be a thing in duels.

It's one of those abilities that has some powerhouse cards that are great in established decks, but not enough to make a deck unto itself.

It's aight.

It's a decent enough keyword in some aggro/midrange decks, but as it's been said, it's mostly an individual-card thingie, not so much a whole deck to build around.

It reminds me of Phasing.

This. There are a couple (dragon, glorybound) that will be fine in other decks. There are a couple that might be fine in Always Watching human decks (the dood who exerts 1-2 CMC doods from graveyard to play, and maybe the 3/3 flyer). There's one Timmys will play that isn't really that great (Piper champion).

However, exert.deck isn't any good yet. Maybe next set could change that, but right now it'd be worse than revolt.deck.

Most of Amonkhet is kind of built as generic goodness suitable for multiple decks rather than specific archetypes.

Exert does combo with setting some traps possibly with instant "untap target creature" cards of which we do have some in this set and the previous sets.
Maybe we'll see some more Blessed Alliance shenanigans for example

What happens if you mix Exert with Vigilance? I'm not sure if we have any in Duels but what happens if there are multiple combat phases and you manage to untap a creature that used its Exert ability, would it be able to activate its Exert ability again for the second combat phase?

The creature never taps, and therefore ignores the negative side of Exert.

That's why I mentioned Always Watching decks.

EDIT: Related to this, on MODO they made it so attack all is no good for exert and you have to individually choose each exert creature that's attacking instead of usual Yes/no pop ups or similar.
How you think they did it in duels? Pop ups probably seem less tedious and matches everything else.
Surprised MODO didn't work same way and maybe even have shortcut to always exert.

After attacks; we'll probably just get a Yes/No prompt come up like we do on Smuggler's Copter.