Since she got announced I knew I wanted to get her. She's a champ with GREAT mobility, great damage, great map pressence and great sustain. Everything a jungler needs. I might not be the best with her but this is what I've tested on PBE on my friend's account and on my own account on LAN. Feel freelly to comment and to critize my guide as it's the first one on here.

PS.- This is a quick guide to starting on Rek'sai so don't expect great explanations

Always start with Q for faster clearing times, follow up with W to recover some health when going to next camp (don't forget to q underground before unburrowing), then go e to start making tunnels (always use it at max fury). Use R to get into lines quicker to make unexpected ganks or to help an ally.

Flash+Smite is the best for any jungler but Exhaust is also another good option instead of Flash. Any other spell is bad on rek'sai.

PROS
Great Mobility
Awesome Ganks
Great Escapes
It's a freacking land Shark
Sustain
Map Pressence
Laughter annoying as **** to the enemy
She's the swaggiest champion EVAH!
CONS
Can't Carry
High cd on CC
Not so good if back
Can't Speak

Basically rek'sai is everything a jungler needs, just learn how to use your tunnels, never forget to q before unburrow and always play Jaws soundtrack when playing.