Just came across this thread and have something perhaps of value to add.
I got one of these hot knives from X-Acto about 35 years ago and it's one of the best tools I've everused for modelling other than a pin drill.
To give a comparison, I was using a Dremel to cut away some plastic and continually had the problem of melt-welding and having to stop every little while to clean out the melted plastic from the cutter tip. It was taking forever and I wasn't happy with the results.
I went to the hot knife and did the same job in a fraction of the time.
The hot knife doesn't get stuck with melt-welding because it stays hot, so the plastic doesn't stick to it. If there is any welding behind the blade, it's almost cooled by the time the wet ends meet so it easily snap off with a pair of forceps.
With this knife, in the past I've undercut all the clothes on the old Airfix 54mm figures, so that you could see up the cuffs of sleeves, the lapels were overlapped as were the front of coats, ties could be separated from shirts and fingers separated to form actual 5 fingered hands. You can almost take a solid block of styrene and carve it like balsa.
On the Revell 1:72 U-boat, I was able to cleanly cut out the long drainage opening between the saddle tanks and the upper casing in less than 10 minutes leaving a nice edge that only had to be lightly evened up with a scalpel.
It was actually easier to thin the plastic out after cutting the slot than it was before cutting it. (recommended if using a hot knife anyway since thin plastic melts too easily.)
The one precaution is that before turning on the knife, make sure the blade is loose in its holder. Otherwise, when the metal expands with the heat, it's almost impossible to get the blade out after, even when cooled.
The blade is easy to clean as the plastic stays wet and is easily wiped off (make sure not to burn yourself)
Also, I let the blade heat up for about 10 minutes before use. It works best when at maximum heat.
