I just got my units last week, so I'm new to this hardware, but I did some testing with water before I actually immersed my Immersion in a more valuable liquid yesterday. I put 5 gallons of tap water in a 6.5 gallon Big Mouth Bubbler with the temperature probe taped about 3/4 of the liquid height (as suggested) and an analog thermometer inside the bubbler to check the actual liquid temperature (it showed that the immersion readout was indeed reliable) I then used the Immersion Pro control to run the temp up to 75° and back down to 50° several times over the course of a week. The ambient temperature of the room ranges from 55° to 70° this time of year, so there was a varied and often significant temperature difference inside and outside the fermenter.
I found that the Immersion Pro was able to heat and cool at about 1°/hr, as advertised. Once at target, it seemed to stay there within about 0.3°. I'd say the temp probe works just fine taped to the outside of the fermenter. I know typical temperature control procedures say that a probe should be immersed in liquid/thermowell, but most of that logic is based on external heating/cooling methods that have a much more powerful output. Because the probe in this system is enclosed inside the substantial insulation of the brewjacket, and since the immersion rod provides a very gentle temperature change from within the fermenter, I don't think that standard logic applies completely.