I haven't used this program in a long time, but it seems that it hasn't been updated for a while, so my opinions are still valid.
Tezxas emulates the ZX Spectrum, a z80 based machine. It does a fair job of this, apparently running at near real speed. However, it suffers from some usability issues. My main problem with Tezxas was that I couldn't read the screen font at all. Because the TI-89 has many fewer pixels than the ZX Spectrum, it had to be scaled down. Unfortunately, this makes the text difficult to read.
Another hardware problem is with the keyboard. I found it confusing, especially the various modes that the keyboard can be in. I'm not sure if this is a function of the ZX Spectrum or the calculator, but the mapping necessary to put it on the 89's keyboard only worsens the situation. Perhaps the author would have been better advised to write Tezxas for the TI-92+, which has a larger screen and a better keyboard for this sort of project.
When Tezxas was released, not much was known about TI-89 HW2 hardware, so the grayscale is flickery. There is a mode to run the emulator in black-and-white, but a better solution would be for the author to update it to use the new techniques.
Perhaps my main problem with this program is that I have little use for it. If I wanted to play ZX Spectrum games, I would probably love Tezxas despite its faults; but I don't. It seems to be relatively stable - the author warns about crashes from the quick-and-dirty style of the code, but I never experienced one.
I would recommend Tezxas for someone who has used a ZX Spectrum before or really wants to; it is not the ideal way to learn to use one.