[tjl]

One big problem with ML is that it's highly based on your training set. There's been a few papers published in computational linguistics that discuss how poorly ML based sentiment analysis is if you try and apply the data to domains outside the training set. For instance, if you train the sentiment data on movie reviews (which is actually a data set commonly used for that purpose) and try and apply it to Twitter or the Web, the results are terrible. But, people keep on trying it.