Firstly the battery cover cracked the second I undid it. There's no way you can over-do the torque on the screws to cause this as they're minute (you'll need jewellers screwdrivers) so the item must have been faulty in the first place.
It sticks to the window and tells the temperature so on that it's reasonable. Haven't compared it against anything else to can't comment on it's accuracy. However as one can assume it's linearly wrong it can still be used as a comparison from day to day.
My main gripe, and this isn't the fault of the device itself more the technology, is that the LCD has very poor contrast at anything below 0 degrees. You almost have your nose on the window to discern the temperature. I also wonder just how long the suckers are going to last in cold temperatures.