From the product page on Amazon.com:
List Price: $99.99
Price: $29.99 Free Shipping for Prime Members
First impression: This network cable tester is light weight but it feels solid enough. It will not be mistaken for a pro level tool but its cost and functionality make it excellent for home use.
Continuity Tester — It functions exactly like you'd expect. The device has an adapter with a couple of alligator clamps that plugs into the RJ11 socket. An LED lights up if there is low resistance or a closed circuit between the two clips.
Cable Tester — It's really just a continuity tester for each wire in the cable, it doesn't do anything to test spec compliance of the cable or terminations. As far as I can tell it only actually tests the cable once when you first hit the "test" button, so it will not track intermittent problems as you move a cable around. It does have LEDs that will indicate some common wiring mistakes.
The manual was obviously written by someone for whom English is a 2nd language. It indicates that it does not test for open circuits in this mode because "Open Circuit is not commonly seen and therefore no indication is included in the Tester." [sic] If by "not commonly seen" they actually mean "very commonly seen and one of the more likely reasons you need a tester" then I totally agree. But it's a moot point. It does clearly indicate an open circuit during testing by not lighting the LED for the circuit that is open.
Cable Tracker — The cable finder works as well as any professional device I've used. The device transmits a signal from the RJ11 and RJ45 sockets. You can also use the alligator clip adapter plugged into the RJ11 socket to track any conductive wire.
When searching for an Ethernet cable, if the signal is weak, you may be able to get a stronger signal by grounding the device. Attach the alligator adapter plugged into the unused port to a pipe or other appropriate ground. While I have found this works, in my tests it was unnecessary because the signal was already sufficiently strong.
This is a great tester for non-professional use, perhaps for a homeowner wiring a house for Ethernet.
Comment: The LEDs are annoyingly bright. Before any further testing I had to paint them over with a Sharpie. Even then I had to keep it pointed away from my face during testing. On the upside one might be able to use it to signal for help after having been blinded. Just try not to blind your rescuers as well.