Pros:
1. It's a pseudo counter. You can air dodge in place to dodge a projectile and grab it, or to dodge an attack, and then zair out of the air dodge to push your opponent away. Works best with ranged attacks since the knockback is good enough to space out your opponent enough to keep them from reaching you.
2. It breaks/knocks away most projectiles. e.g. missiles, turnips, link's bombs*, boomerang, snake's grenades, small charge shots, small aura spheres/shadow balls, pills, fireballs, pikmin...
3. Incredibly safe edgeguard tool. It breaks all recovery moves. ALL of them, so far, that I've tried. The only time it won't hit anyone out of their recovery is when it won't hit lol.
4. It can poke at shields, or stun opponents long enough for a missile to hit.
5. It's safe at a distance, whether your hit or not, with most characters.
6. It disrupts your opponent's rythm.
Cons:
1. Can be punished by some characters with strong crouch cancels, like Bowser.
2. Can be countered by Link's bombs. He can block the zair and jump out of shield and throw the bomb at you before you can recover from the Z-air.
3. It does incredibly low damage. It's not a move to rack up a lot of damage. It's simply a really strong keep-away tool. 7% first hit, 4% all following hits in the same stock. Resets every stock.
All in all, I'd rate it a 9 to 9.5/10. It's got very few cons and has a vast usability. It's the one tool she's been missing since Melee. Learn the timing on it. You can change its length/speed based on how high/low you air dodge/perform the z--air.