Pros:

Good early

Can snowball really hard

People don't know how to fight it

Excellent Splitpush


Cons:

Feast or Famine

Hard matchups are HARD

Can be camped easily

Suboptimal (just play renekton)

[Don't forget to read the notes, alot of things are explained there]

Your goals in the laning phase when playing Rek'Sai top:

Get fed.
Get the tower.


What you wanna try and do is get your level two before the enemy, like basically every other bruiser.
Don't be afraid to use your Q to do so, as the cooldown is really low, and you don't use mana. But watch out! you don't want to push the wave too far, else you won't be able to engage the enemy laner.
If the enemy laner gets too cocky and goes in deep to get the casters, just smack him a bit with your Q, should get them to back off a bit.

Once you get level 2, you can start trying to look for engages, stay burrowed for as long as needed, and try to farm with your burrowed Q. If the enemy is smart, they won't approach until you unburrow to farm, so try to make them lose more cs than you using your Q or preping the wave in advance.
If you manage to get under them burrowed, just knock them up and chunk them, preparing them for your level three.

If all goes according the plan, getting to level 3 should land you a free kill.
It goes like this: Burrow > (Burrowed Q)> Tunnel > Knockup > Auto > Reset w/ Q > Q > Q > E (> Burrowed Q)
Feel free to ignite at any point during that, gives you a free kill, show 'em who's the queen.
At this point, you should probably back depending on how chunked you are. If the enemy is really weak (kayle or sylas) and you have wards, you can stay in lane for another kill. (If you get another one you should really back)

Once you get Tiamat, Rek'sai Q enables you to destroy their tower in two go. Not even kidding, it's that strong. (One go if your Jungler gives you herald.)

Assuming you got their tower, you should try to make things happen mid (or bot if you took TP).

Depending on the state of your team (i.e if you're not the only carry/if you have TP), you might wanna try and draw pressure top with your insane splitpush while your team does drake or takes picks. If you're fed, you might even get some more kills up there.

Assuming the enemy team had the time to scale for whatever reason, splitpushing isn't that great of an option except if you have TP. You need to either get picks in 1v1, or dive their ADC/carry from the fog of war, and let your team handle the rest. (Rek'sai's lategame isn't the greatest)
If for some reason you can't manage to kill their carry, resort to peel for yours. (You can go tankier and/or take a Knight's Vow)

I might or might not do this chapter, most of it is explained in the notes anyway.