Product review (5 stars): Acu-Rite Weather Thermometer Model #00611A2
The reviews were all over the map on this item, which caused us a bit of concern, but we found similar types of reviews on many other indoor/outdoor thermometers as well.  Because this particular model was reasonably priced and it had a nice readable display, we took a chance and it works great. We left both the inside unit and the outside sending sensor next to each other for about a half hour after initially installing the batteries to see how the temperature and the humidity readings would compare.  It turned out they were within one degree of each other on the temperature and within two percentage points on the humidity.  You aren't going to get much closer than that on a unit in this price range.  The installation of the outdoor sending unit was a snap.  One small screw had it quickly attached outside of our bedroom window. However, it's designed so you can also hang it on a string or wire if that is your desire.  We have the indoor unit about 15' from the outside one and there has never been a problem with the wireless communication.  The indoor unit sits on a base which is used to support it in the upright position on a flat surface, but you can also install it on a wall.  We had a little difficulty separating the base from the indoor display unit, which you must do in order to get to the battery compartment.  The instructions show a simple "slide off" method, but there are a couple of prongs that tend to stick.  Although we eventually got it off, it took some effort.  This should not be a deal breaker in considering the Acu-Rite, but since we had this experience, I thought I'd mention it.