Scroll down to post #6 on the following thread to see a textbook US Midwest image from NEXRAD (the link in post #1 is dead, and as its poster indicates the photo is an XM satellite paint that filters out weak returns, including gust fronts most of the time). Have never noticed one on airborne radar (thank you Lord for small favors), and if anyone has, would be interested to know at what altitude / tilt setting / etc it was observed.
Nice example of a gust front along a squall line - Pilots of America Message Board