ZTris was the first Tetris colone I got on my calculator that actually worked. It's... well... tetris.
There are a few things that I'd like to mention, however.
There is no holding (yeah call me a wuss, but holding is really useful sometimes especially when you're going on an epic rampage during class)
The pieces are solid black, which annoys me. Sometimes you can't tell how many squares across something is.
Controls, calculator wise, are excellent-- the only problem is that most other platforms use up as drop and A or B for rotation. Whenever I play on any other platform I get screwed over for the first few rounds becase I'm too used to these controls.
Otherwise (besides the lack of ghost pieces, which I won't go on about since i don't even know how you could do taht on a calc), its a pretty good, faithful clone with no bugs, 100% user friendly, and blahblah blah