Pretty good, but only if you picture is..... – Twixt Time Review

On the average, the app works pretty well. I am tracking 4 different timepieces, all with automatic movements. Once you have a few entries for that timepiece, it averages out to right at what I was calculating by hand, using an atomic clock. The most difficult part is in taking a quality image of the face. You need even lighting, with no glare (like from a lightbulb), and you need to hold the phone "square" to the watch face. You need to be able to see every minute market, regardless of how many you have, with equal clarity and they must all be at the same angle. If you do not accomplish this, the "12:00 marker" will not line up properly and you will notice that the lines for the hands do not fall perfectly in the middle of each hand. If these do not happen, you will receive inaccurate measurements. Should you proceed, despite the problems I described, it will require several measurements before the average works out to be accurate.
Review by Wm Gardner on Twixt Time.