Product review (4 stars): How to convert these to display directly in Fahrenheit
The biggest complaint most people have with these units is the Celsius readout.

But the circuit board has a location for a jumper (solder blob) to be added to switch them to Fahrenheit readout.

Opening the units is probably the hardest part of the whole procedure.

Slide the battery compartment open and remove the two button cells.

Get a small flat-bladed screwdriver and insert it into each of the four holes on the back of the unit, one at a time, working your way around, and pushing the little latch tabs inward until you can disengage the tab from the hole it locks into in the front part of the case.  Once you get them all loose, you can separate the back from the front.

As you remove the back, notice the orientation of the circuit board within the front half of the case.  Notice also the "zebra strip" that connects the display to the circuit board when the unit is assembled.  These zebra strips have alternating slices of conductive and insulating "rubber-like" material made into a sandwich.  You can see the alternating black and translucent slices if you look very closely.

Anyhow, the zebra strip is rubbery and conductive at every other little slice.  The idea is that a row of contacts on a circuit board can be connected to a corresponding row of contacts on an LCD by simply squeezing the zebra strip between the PC board and the display.  So when the unit is assembled, the PC board is pushed down against the zebra strip hard enough to squeeze the zebra strip into good contact with both the display and PCB.

Now, when you pull the PCB completely out of the front piece, and turn it over, you will see the row of contacts along one edge, that are meant to connect to the zebra strip.  So it's easy to get the board back into place the right way by observing those contacts and knowing that they must be in contact with the zebra strip when you put it back together.

OK.  Now, look at the green side of the PCB.  You will find a set of three contacts near one corner of the board.  One is marked "F" and one is marked "2S".  The third (middle) contact is not marked.  It's presumably the common for the other two.

By bridging across between the middle contact and the one marked "F", the display will read in Fahrenheit.  The idea is for you to just put a blob of solder in place to bridge the two contacts.  That's easily done if you have any experience soldering circuit boards.

The other contact, marked "2S" probably speeds up the rate of update for the display.  That would cause the unit to read quite a bit faster, which might be good in some applications.  However, it would also probably reduce the battery life.  So that would be a trade-off you'd have to decide if you wanted to make.

I haven't had time to fully test the units I got, but they both read out to within 1/10th of a degree C of each other, so that's encouraging.  Then again, they may both just happen to be off the same amount in the same direction!

I will likely test them and maybe even see if there's a way to calibrate them by changing the value of one or two of the resistors on the PC board.

But in the mean time, I give them four stars for being pretty good for the money.

Edit to add:

These read the same at temperatures near room temp.  For example, now that I've modified them both to read in Fahrenheit, they're on my desk here, and both reading 74.3 degrees F.

But I tested them in a properly prepared ice slush, and where they should have read 32.0 degrees F, one of them read 34.7 and the other read 34.3.  So they're both off by several degrees F, and not identical at that temperature.  They may well be off at room temperature, too, but just happen to agree with each other.

Further testing, and some investigation into whether they can be calibrated is warranted.