The code I used to participate in Kaggle's Yelp Recruiting contest. Find my profile here.
Yelp wanted to be able to predict what reviews would be deemed useful by their user base before those reviews were made public. They packaged a bunch of their data up for this competition. I spent a lot of time learning about different strategies for sparse text vectorization and what scikit-learn algorithms could run on that sparse input. My final model focused on optimally using the vectorized text which the winners then revealed didn't contain much useful information.
I added unique identifiers to each review text in order to add information about the business category. I then used a hashing vectorizer on the review text. I applied two models to the sparse, vectorized text: a random forest regressor and a stochastic gradient descent (SGD) regressor. The SGD regressor was able to handle the sparse input. The random forest regressor was not, and so I found as many cluster centers as would fit in my laptop's RAM and calculated each review's distance to those cluster centers. That was then the data that I fed to the random forest regressor.
I used the output of each of those regressors in a new random forest model along with user and business information (number of previous user reviews, average business rating, etc...). That was then the final predictor. It turned out that people had the most success creating features from the user and business information and only including general description stats about the review text (length, number of paragraphs, etc...).