I'm going to put in my vote for *not* doing this, and here's why:
By dropping the sensor into the beer, you're basically plugging the controller into something with a fairly large thermal inertia; the fridge/freezer will be dropping the temperature of the air, waiting for the beer to catch up. The problem is that it takes a heck of a lot of energy to drop the beer temp, as opposed to the air temp.
So, by the time the beer temp hits your setting, your air temp will be *much* lower than that, and it will continue to "carry-over cool" your beer, many degrees lower than you intended.
I lost several batches of beer, this way, as the over cooling slowed the yeast down so much that it dropped out and went to sleep before it finished. My beers were green apple sour-bombs, due to the fact that the yeast never finished converting the acetaldehyde to ethanol, before they dropped out.
Tape your controller to the side of your fermenter; cover it with a chunk of insulation or styrofoam, and your fridge/freezer will cool the beer in shorter "steps" and ensure you don't way undershoot your target temp.