OK it isn't bad, especially considering the price!
It runs a freewheeling record cycle as indicated by a colored "pause" button. 1.5 seconds of red while recording, then green for display of info, then yellow for standby and back to red. You can hit the color-changing pause button to keep displaying the data or toggle a save/clear button keep the last note played as a reference. When a note has been saved as a reference, the next time a note is played the displayed data also includes a bar graph of the beatrates of the partial matches. This bar graph stays displayed even though the program continues freewheeling through the recording cycle although other data, like the number of cents all the partials deviate from theoretical, is cleared from the display (unless you hit pause), until you play another note.
So what you do is clear the saved reference note, wait for the button to turn yellow, play the first note of an interval as the reference note, and toggle the save/clear button. You can then play the second note of an interval, about when the button turns yellow, and get the beatrates. Then adjust that note, or play another note for another interval to the same reference note. What you cannot do is play both notes of an intervals together and find out what the beatrates is.
The beatrate bar graph has division that should let you read to 1/4 bps. There is no digital beatrate display that I saw, although you could pause and write down cent deviations of the partials (to 0.1) and compute them.
For me, it will be most useful for objectively evaluating aural tuning sequences, starting with the 4ths and 5ths 12th sequence I am using. It is one thing to say "tune the 4ths and 5ths such-and-such a way and such-and-such M6 will beat such-and-such" and objectively verifying this in the real world. It could also be very good for training the ear to hear beatrate equality and progression. There are many other graphic features, too.
I did have a problem getting the serial authorization number, but it was on my end. The service I was using put the email in "quarantine". I didn't find out until after I contacted the application's publisher, who responded very quickly with the authorization number.
I am going to get a better mic and play with this some more. For $5 bucks, yeah it's worth it if you already tune aurally and want to verify beatrates, or want to learn to tune aurally and need to learn beatrates. I haven't ever used a ETD, so I can't say it would be a good or better or worse way to tune electronically.
Who taught the first chicken how to peck?