Comparisons of Cellular Service Quality?

dmr asks: "What are the best sources of information for cell phone service quality? Is there a source out there which can give a reputable ranking of service, coverage, dropped-calls, etc, for the major cell phone providers, broken out by region? There's a huge pile of anecdotes and tall tales out there. Where can someone find out which providers work best in his hometown? Bonus points for assessments of capital improvements to equipment, as well as trends from the past."Update: A commenter points out that MyRatePlan has a great coverage map comparison tool.