This is simply the best Tetris clone I've ever seen. It is pretty much exactly like the Gameboy version - pure, unadulterated Tetris goodness.
Features for increasing speed, an option for a height handicap, and anything else you would expect from a proper Tetris game are all included. The great 2-player mode includes line penalties if your opponent clears multiple lines, which can be changed on the main page from scrambled or linear. It also has a basic height meter, so you can know roughly how your opponent is doing.
The interface is clean, polished, and well spaced out. What I found pretty neat was that after a certain amount of lines cleared, you don't only raise a level of difficulty, but also the stage background changes. The only real complaint I'd have is about the block graphics being pure black. Sometimes it can be hard to judge distances since the squares can't be distinguished, especially at high speeds. Also, at a quick glance, some pieces look similar and can throw you off. This is a little nit-picky, though.
Controls are one of the biggest reasons why this is a great version of Tetris. "Ztris" has everything you may need, two-way rotation, movement, instant drop, pause, and quit (plus MirageOS's functions such as returning to the games menu, exiting it all, or turning off the calculator completely) all of which are quite intuitive, even if you don't believe in reading readmes. Especially at higher speeds, the difference between a single flip and 3 can make a lot of difference, as well as "skill" flips in tight spaces which require the right direction.
Overall, I'd give "Ztris" an 11/10, for simply being way too awesome. This will be on my calculator until the plastic warps, rusts, and rots.