USA Today quoted a recent InsiderFlyer research on frequent flier miles redemption availability. Contrary to common perception that such awards are difficult to get, InsiderFlyer found out that free economy-class seats are available 73% of the time, and business-class seats 54% of the time.
Details by Program:
Economy-Class Seats:
Delta (SkyMiles): 85%
American (AAdvantage): 80%
United (MileagePlus): 79%
US Airways (Dividend Miles): 67%
Northwest (WorldPerks): 62%
Continental (OnePass): 54%
Business-Class Seats
American (AAdvantage): 92%
Delta (SkyMiles): 80%
US Airways (Dividend Miles): 65%?
United (MileagePlus): 40%
Continental (OnePass): 32%
Northwest (WorldPerks): 15%
InsiderFlyer's own coverage of the study can be found here. It also includes some more findings, like OnePass's and Dividend Miles's call centers are more successful finding award deals than its online peers, while Northwest's online arm offers more choices.