Rek'Sai is a fundamentally flawed champion.
Before I say anything, go check out the champion spotlight for her below:
https://www.youtube.com/watch?v=bmurGIqiPgg
Have you noticed a theme? They try to sell her as this sort of fighter-esque champion in a similar vein as Yasuo and Fiora, but in my experience, most of her kit is counter intuitive to that design. The problem lies most prominently in her W. This ability is supposed to be the pinnacle of her kit; it represents her identity and her strength as a mobile fighter (supposedly). Unfortunately, her W/E is way too slow of an initiation maneuver in of itself and it's way too easily telegraphed. If you try to build her like an AD fighter, you'll either get shoved away as you meagerly crawl in their general direction to try to knock them up, or you'll get blown up in two seconds upon arrival. The only way to have a successful entry is to build full tank, which is how people have been building her as of late.
You only need to look at her typical build path in order to see how bad this design choice was.
http://puu.sh/qNj4N/4f13d722e6.jpg
It's an insult to her identity. She's supposed to be this master of burrowing and sands, and she CRAWLS towards her enemies like a damned caterpillar when she wants to engage onto somebody. Not to mention that it's not even a good CC ability anyway. There are plenty of other engage junglers who do everything Rek'Sai does, only better and with way more utilities at their disposal, such as Malphite, Sejuani, Cho'Gath, Zac, Sion, Jarvan, etc. For what it is, its cooldown in order to repeat its knockup is way too long. She can't be classified as a damaging jungler because she lacks so much in DPS that others like Shyvana, Lee Sin, Olaf, Vi, Elise, etc have intuitively in their kits; she is heavily item reliant in terms of damage, but she doesn't scale well for the purposes of being terrible at getting near her target.
Her W/Q is useless in its intended purpose as a "scouting tool" because it only adds onto that weakness as being easily telegraphed. If you manage to hit somebody with your Q, they'll know you're there and quickly react to your imminent approach. If you miss, but they're still there, they'll see your projectile and probably make an attempt to counter you since your vision is so minuscule. The UI for the tremor vision is unreliable and hard to see, so you'll largely rely on wards or your team for vision if attempting to engage over walls, which is dumb for an intended "surprise" champion and counter jungler.
The rest of her kit is pretty OK, if not totally bland. She needs increased utility or to have her W offer more of a reliable engage tool. Some changes I propose to her kit are as followed:
Allow her W/Q projectile to "explode" at the end of its travel in a sonar-like radius and provide vision to that area for a little bit of time similar to Ashe's E.
Increase the movement speed of her W while burrowed and significantly speed up the W/E travel speed.
Make her immune to CC during W/E.
Have just one of these changes happen would drastically improve her playrate and usefulness, but until then, she's just a wannabe Shaco that drops off harder than a 0/5/0 Pantheon.