The thing works by reading the infrared emissions from the ear drum.
The ear drum is hotter than the ear c{*filter*}walls, so you have to point
the thing very carefully.  This means tugging on the top of the ear
to straighten out the ear canal, then inserting the thing snugly, then
pushing a button.  Unfortunately, there are many things that can go wrong.
It is almost impossible to aim the thing correctly when you do it on
yourself.  I get readings which differ from each other by up to 2 degrees,
and may differ from an {*filter*}thermometer by up to 2 degrees.  

I talked to one of the nurses in my doctor's office recently about this,
and she said she didn't like them either, for same reasons.  She did give
me some instruction on how to tug on my ear, and what correct insertion
feels like, but she said she thought it was impossible to do correctly
on one's self.  She also said that she and other nurses had complained to
the company about inaccurate readings, and that someone from the company
had told them to take great care to clean the infrared window at the end
of the probe with {*filter*} from time to time.  She demonstrated this prior
to reading my temperature, and managed to get a reading within 0.5 degree
of the {*filter*}temperature I took at home before driving to the Dr's office.

I have also noticed tha some nurses click the button, then remove the
probe immediately.  This causes wrong readings.  In my experience, you
have to leave the probe in a good 1 to 2 seconds after clicking the button
to get a good measurement.  The nurse I talked with agreed.  I suspect
that many people don't realize this, and therefore get bad readings for
yet another reason.

In short, it's a great idea.  It may work for some folks, but I believe
it doesn't work well for a person who wants to take his own temperature.