Graphics - 3/5
While the graphics are nothing special, they stay faithful to the original game for the Game Boy and NES. My only gripe is that the graphics sometimes look slightly ugly for the level backgrounds. I am aware that in 2000, multi-shade grayscale was probably really hard to implement, but it still doesn't look great.
Gameplay - 9/10
ZTetris is just as addicting as the official Game Boy version. The 10 levels each give everyone something. Are you new to the game? Level 0's for you! Intermediate? Level 5 or 6! There's something fore everyone in ZTetris.
Miscellaneous - 4/5
The UI is intuitive, the leaderboard is standard yet fun, and the 2-player mode is an intriguing, innovative idea.
Conclusion - 16/20 (8/10)
ZTetris is one of the quintessential games for a TI-83 Plus. I can see why it's one of the 100 most downloaded programs across all platforms. It isn't a masterclass in anything, nor is it a new kind of game. It just takes the tropes used before and uses them really well. It may not be perfect in everything, but it isn't bad or even average in anything either.
All in all, ZTetris is a commendable game, meaning it gets an 8/10.