Rajiv Sethi has been assessing the performance of prediction markets:
Forecasting Elections: This wild and crazy election cycle is generating an enormous amount of data that social scientists will be pondering for years to come. We are learning about the beliefs, preferences, and loyalties of the American electorate, and possibly witnessing a political realignment of historic proportions. Several prominent republicans have vowed not to support their nominee if it it happens to be Trump, while a recent candidate for the Democratic nomination has declared a preference for Trump over his own party's likely nominee. Crossover voting will be rampant come November, but the flows will be in both directions and the outcome remains quite uncertain.
Among the issues that the emerging data will be called upon to address is the accuracy of prediction markets relative to more conventional poll and model based forecasts.
... The results are in, with Cruz taking Kansas and Maine and Trump holding on to Kentucky and Louisiana. The only missed call by the prediction markets was therefore Maine. Still, the significant margins of victory for Cruz in Kansas and Maine suggest to me that traders in the aggregate continue to have somewhat inflated expectations regarding Trump's prospects. And I'm even more confident than I was early this morning that Rubio faces a humbling and humiliating loss in his home state of Florida, though he may have no option now but to soldier on.