If you ever move to glass carboys, I would advise using a thermowell if you really want to be able to control things precisely. I use a chest freezer for a fermentation chamber as well. I've noticed that the freezer cools the glass much more quickly than the liquid in it. If the thermowell is taped to the glass, it's measuring the temperature of it, not necessarily the beer.
As long as your probe (and the carboy glass near it) is insulated with bubble wrap or foam, I'd think that it would still be pretty similar to the internal temp of the ferment?
I always thought so too, but I've found that it's not, in my set up anyway. I've used both bubble wrap and foam rubber to create a pocket over the probe to insulate it from the ambient air, but it still gave a much different reading than the thermowell when the freezer is actually cooling. The freezer is cooling the glass very quickly which then cools the beer. The thermowell is reading the temperature of the glass, from what I can tell. With a plastic container, I don't think it would be as much of an issue, but it is with glass.
Since I've moved to the thermowell for the "cooling" probe, I've been much more pleased with my beers. I also run another Ranco attached to a fermwrap for heating. That probe is attached to the outside of the carboy, so I can see the readings for both locations at any given time. Everyone else's experiences may vary, but I didn't find that taping the probe to the outside of the carboy was as accurate as I would like. A carboy hood with a thermowell through the top has been much more accurate for me.