Has anyone had first hand experience with the Zeiss thermal add on, and how do they compare to a dedicated thermal scope.
May I ask why are you interested specifically in Zeiss.
There are other more known brands in thermal hunting area, with much more developed and tested devices.
If you are looking for reliable thermal front add on, just pick either Pulsar Krypton 2 or Hikmicro Thunder 2.0.
Pulsar PSP-B adaptor is good and reliably holding zero even when I mounted and dismounted it on several different rifle-scopes. I have tested it on 223, 308 and 30-06 calibre rifles and it never shifted POI.
I did not have a chance to test Thunder with original Hikmicro adaptor, but it works brilliantly with Smartclip adaptor on my 223.
During the testing time I have swapped adaptor and eyepiece on thunder several times and each time when I reattached adaptor, I went to shooting range and tested it. POI always stayed the same.
If you have quality daytime glass scopes and want to keep them for day time shooting than front thermal add on devices are great solution.
But if you want to have dedicated night time setup than pure thermal scope is probably the best solution. Thunder 2.0 TQ35 or either one of Pulsar Thermions. Depends on your budget and typical hunting scenarios.
Cheers