I got one of these last year for tuning Bluetooth antennas. Getting one was a bit of a struggle due to a long lead time. At the time I presumed it was due to popularity.
The device and the software all looks impressive and appears to be amazing value for £400. However.........
Post-sales support is totally non-existent. Once Martin Nirschl has your cash, that's the end of it.
Since the device uses broadband detectors, it is incredibly thermally sensitive, even though it claims to be thermally compensated. You need to put it in a thermally controlled chamber if you want any chance of re-using an old calibration file. It also takes a very long time to stabilise thermally. I insulate the case for a while when it is first turned on then don't bother to do any measurements until it is near the temperature when I made the cal file. If it's a hotter day than when you took the cal file, you'll have to cal it again.
Although it claims to have a wide bandwidth, when you run a plot or a calibration, there are big holes in the frequency plot where the results are just rubbish. It is also very slow. Too slow for live updates while you wiggle, move or change bits of your circuit.
Of course, you get what you pay for. It does sort-of work for basic measurements but you have to be very careful, particularly when working near the circumference of the Smith Chart and trying to do impedance matching, because of the limited dynamic range and the thermal sensitivity. If you are doing anything serious and you can complete your job in a reasonable time frame, you'd be better off hiring a proper VNA for the same money.