TL;DR: A pretty fun but flawed game, Tetris 99 is great at what it does, but not much else. The battle system is really unique and works surprisingly well, the gameplay is the snappy and precise Tetris we know and love, and the different collaborations for new themes and music is a great touch that makes the game interesting enough to fire up again every once in a while instead of some other version of Tetris. The two major problems are that
1. You barely get anything in regards to tangible rewarding if you don't place 1st or at least the top 5-10, and
2. Almost everything aside from the game's main mode is paywalled behind a 10€ DLC. I'm not getting that, I'd rather get a full Tetris game on sale for that money!

A solid game to try out if you have Switch Online though, it can't hurt either.