Nate Silver's initial reading of them:
The most important ground rule is that the forecasts are based on one type of information and one type of information only: state-by-state polls. You are absolutely welcome to consider other objective and subjective factors in addition to the state polls. We will continue to discuss those factors on this blog. We think these forecasts provide a valuable perspective but that doesn’t mean that you should ignore everything else. However, building a well-calibrated forecast from the polls is challenging enough given the uncertainties inherent in primary polling, and so that’s what we’ve decided to focus on rather than anything fancier.