Rek'Sai is okay, but might require a tweak.
After playing several games as Rek'Sai in different scenerios (I have not tried top Rek'Sai yet), she is good but might need a few tweaks to make her solid.
Her kit is fine, theres no need to change any of her skills conceptually. Though the only two changes that would be good would be (someone stated this earlier) to have a slow on her burrowed Q because her bonus movement speed is not really much at all, it reminds me of Elise in spider form. You could lower the range on her (AOE nidalee spear) Q, and add in a slow. She would be very viable as a gank/chase type champion. Because I've had so many moments where they would run away from me, and if I somehow ended up using my E as a gap closer they could either still walk away or walk in the opposite direction of where I am digging, making myself even farther from my enemy. (I wish you could aim how far to travel between holes. Probably less than maximum.)
The second change is if she isn't considered to be much of a chaser/ganker; could she become a bruiser? Well, all I know is that from my experience that Rek'Sai, gets wrecked really fast in a fight. I buy omen, sunfire, and other hp + armor items, yet I feel like the adc shreds right through me. I could see her as a fighter because of her fury gauge. Her E is powerful when her fury is full, if you activate W you end up expending fury. To trigger the true damage on her E you have to reach max fury, which is difficult because she gets melted pretty fast as a melee. So, which is it? Is she suppose to be a bruiser that excels at long fights? Or a chaser/ganker? It feels like Riot was trying to mesh these two types together and what we got was pretty much half of each type. If Riot wanted to make her a bit more of a champion that fights in the center, using her unburrowed Q to deal moderate amounts of AOE damage while building fury, she would have to have a stat boost in her armor and mr. Or anything to make her a bit more tanky.
Yes I also did try AP Rek'Sai, its not that good. Sure, if you can last until mid to late game, you'd have a lot of fun spamming Q and dealing massive AOE damage every 4 seconds. But you put your team in a lot of stress for the early portion of the game. Its like you're experiencing a 4 v 5 for the first 15 minutes of the game and hoping that your team won't die to much.
Psst. Try AP Support Rek'Sai, the fun starts at level 2. Along with many knock ups for days. Still a terrible idea but I've had great success with it.
I tend to build appropriately in response to the enemy's composition.