Alex Eames has conducted a series of field tests (okay, not in a field exactly) on three wi-fi dongles: the official Foundation dongle, a standard Edimax dongle and the dongle from The Pi Hut. He tried them out in different locations throughout his house in Poland and recorded the signal strength and power usage for each dongle in each location. He then carried out analysis on the results and has presented them in great style on his blog at RasPi.TV. The results are very interesting and really give you a sense of which dongle performs best and which dongle uses the least power. Read the blog post here.