Saw this posted at the evil empire, and decided to liberate it, plus do some research. Here's my take:

This was actually pretty dang cool for the time, letting a person jam along to accompanying music (not unlike karaoke) using only some... D cells to power the whole unit, I believe. Or one could simply use the tape recorder to grab guitar recordings for later playback. So-- good for quick demos, testing things out, and transcribing stuff on the fly, all in a very portable, lightweight form.

CONS:

• The built-in speaker certainly did sound like crapola.
• The neck was professional-grade, along with (arguably) the electronics, but the main body was made of thin-ish plastic that didn't exactly inspire confidence in heavy / casual usage.

PROS:

• This sucker had a fairly impressive range of all the core digital adjustments a pro would want, such as mixing levels, mixing volumes, basic effects (like whammy & overdrive), etc.
• The player was NOT actually stuck with the cheap internal speaker. The EG-5 had 1/4" (amp!) and 1/8" inputs & outputs, as well as DC power input and a couple other thangs. (see lower-right in the shot below)

Here's a magazine review:
https://www.guitarplayer.com/gear/the-plug-in-and-play-performance-of-the-casio-eg-5-cassette-guitar-was-pure-1980s-weirdness

And here's me laddie, checking it out crown-to-bum, playing it a bit:
https://www.youtube.com/watch?v=IwL2nPMIcgw