I have owned the Dekunu One for approximately 1 month and have got about 15 jumps on it so far. My experience so far has been positive. Prior to buying it, all I could find were negative reviews but I decided to take a gamble and see for myself.
The device provides a huge amount of data to the jumper during all phases of the skydive (Flight to altitude, Freefall, Under Canopy). The information I love is the data displayed on the flight to altitude. It lets me know when the plane is on the jump run and how far from the DZ you are. This information especially useful on those cloudy days and helps confirm what your eyes are seeing when checking the spot.
Under canopy, it gives you the distance to the DZ and direction, ground speed, and your altitude, which could be very beneficial if you find yourself caught in the clouds which can be the difference between making it back to the DZ or landing off.
The only two negatives I have at this point are the battery life and having to switch modes (Belly, Freefly, Hop & Pop). It will probably last one day of jumping if left on from the time of your first jump until the end of the day. I turn it on to get a GPS lock once I arrive at the DZ and then turn it off until about 10 minutes before I board the plane. I made 7 jumps in one day and still had over 50% batter life doing this.
The Dekunu Could also provides a ton of data that can be visually displayed in 3D which is pretty cool.