I've not tested them in the application I bought them for, as I ended up running a separate 5V mains SM PSU inside my HiFi amp, but I have now tested one of these on the bench. I put a 1k load on the output, (as they need a load), and looked at the output voltage on my scope, and I didnt see any especially nasty noise.
Although the device is the 5V version, it seemed to output voltage as soon as the input was around 0.5V.
One thing to note is that the output is unregulated, and for my 1k load ended up being 5.5V when the input was 5V, but this may be because the 1k load was too low, as the spec reads
To ensure this module can operate efficiently and reliably, During operation, the minimum
output load is not less than 10% of the full load, and that this product should never be
operated under no load!
Its rated at 1W (@5V) so the max operating current is 200mA, so 10% is 20mA, so should have used something like 250 Ohm as the load.