I actually really like Rek'Sai
I bought her right when she came out, and I've played a few games with her in normals, ranked 5s and even solo queue. In all of these modes, I felt her to be a great pick that helps the team a ton while also having serious solo carry potential. She feels great, and this is the first time since Yasuo that I instantly picked up and loved a champ.
I really enjoy the concept of the tunnels, and how much mobility they can provide while also not being some point-and-click escape. (Should I make a Lee Sin joke?) They can be used early to help you move around your jungle faster to stack up the devourer, and can be used to get under someone quickly for a knock up during a gank. In the later stages of the game, they can be placed offensively in the enemy jungle and near objectives to be places you can ult, and places where you can dip in and out of an area or even a fight quickly. Going from one side of your jungle to the other at mach speed is incredibly satisfying.
Her burrow mechanic is really fun to use, and the tremor sense that comes along with it is a unique way to stay safe while invading or in your own jungle. Also, in a teamfight, I love being able to start a fight, then quickly dip out while regenerating some of my health before popping back from underground to fight again.
I do see some problems though, mainly her Q in her burrowed form. I know this is a way to gain vision of someone you've caught with your tremor sense, but I think this needs to have some stickiness or another mechanic on it. Probably not a slow, because that would make it too good for escaping. Maybe some speed boost mechanic while moving towards and enemy tagged with it (like vayne passive.) I would talk about her early game damage being a bit overwhelming, but compared to the other fighters in the game, it isn't too high.
That's pretty much all I wanted to say. GJ Riot for releasing a fun and engaging champion that is quite tactical with map pressure and requires some degree of skill, while still not being overloaded.