I have the one linked in the above post, $15. It's not so good. Firstly, the instructions are horrible, they are a translation from a foreign country. Second, the calibration screw is deep-set below the plastic case and embedded in a circuit board. You have to fish the tiny screwdriver through the hole, hoping it lines up with the screw (and usually doesn't until the third or fourth try). Because the screw is on the back and precariously located deep in the case, you cannot adjust it while looking at the readout on the screen. You turn the driver a little, then turn the unit around to see what change has been made on the screen, lose the position of the screwdriver, and start all over again. Calibration of the unit is miserable. But it does seem to work and I'll keep it and use it. If I had it to do over again, I would go after the more expensive self calibrating device by Hanna.
I do prefer using this device to the phenol red color match. I know some folks say the color match is fine because you're just trying to keep the pH in an acceptable range. But a range does nothing to help you determine how much acid to add using the Pool Math calculator. If we are striving to do what TFP guidelines recommend, a range is not good enough...for me.