Not very useful at all. To accurately measure temperature you really need a Stevenson screen (http://en.wikipedia.org/wiki/Stevenson_screen). But furthermore, this needs to be located in a good location with attributes such as good air circulation, lack of buildings, concrete, trees, etc. It's often impossible to meet all these conditions in an urban area, which means you're really measuring a micro climate. While there are some sites that crowd-source data from personal weather stations, there's really no guarantee that people have followed good guide lines with the placement and installation of their weather station. It's sometimes obvious when this is occurring because you see some pretty large variations in temperature in more or less the same neighbourhood.