It's a good product aesthetically, but doesn't work if the gap between the magnet on the rim and the sensor on the fork is more than 5mm. That's a very big drawback. This is a big No for MTBs, it's just meant for city bikes
I'm quite happy with this - I wanted a simple little bike computer to give me an indication of speed and distance. It's difficult to quantify whether you're getting fitter/faster unless you actually have an idea of how fast and far you've managed to go. I didn't need anything complicated or expensive like the colour screened Garmin Edge, so this fit the bill perfectly. It does km/h and m/h, and measures instantaneous speed, avg. speed and route distance, with a total history distance that persists, and can be set for a number of pre-calibrated wheel sizes. Seems fairly accurate compared to a GPS (need to do a long straight stretch to compare with a GPS, as a GPS based cycle computer will average distance over time to give speed).
One thing worth mentioning - the fork part of the sensor doesn't fit brilliantly on fairly flat, aerodynamic carbon forks. It fits okay but I do need to check it every now and then. It would suit a more rounded 'classical' fork design like a hybrid or touring bike better with better purchase on the fork.