Just come back from Thailand 2 days ago and as promised, here are my take on the Phottix GPS: overall I'm very pleased with the results.
As with all the other GPS, after switching your camera on, it will take a few seconds to acquire signal from satellites (the GPS blinks red during acquisition and there is a blinking "GPS" indicator on the D600 rear LCD).
It is sometimes longer but usually it takes no longer than the time to take a first shot and adjust my exposure (I shoot mainly in Manual mode). When the signal is locked, the GPS indicator will be fixed green and the GPS indicator on the D600 will stop blinking as well.
When the signal is locked, you can switched the camera off and when you turn it back on, the GPS signal will be set within just a few seconds if you have not moved too far away from your previous position of course.
I've been shooting on different islands in the Adaman sea, sometimes quite small and the GPS coordinates appear to be fairly accurate.
As said in my first post, build quality is good and I have nothing to report yet. Overall, for EUR100, I'm very happy with it.
PS: although it is not listed, this GPS is compatible with the D600.