Here is some information from their website:
The core of the dataset is the feature analysis and metadata for one million songs, provided by The Echo Nest. The dataset does not include any audio, only the derived features. Note, however, that sample audio can be fetched from services like 7digital, using code we provide.
The Million Song Dataset is also a cluster of complementary datasets contributed by the community:
Here is information on getting the dataset. Kaggle managed a contest for rating music items drawn from this dataset. For evaluating performance they used MAP@500 metric described here. Anyway I am soon going to try out our GraphChi CF toolbox on this dataset. Keep posted for some results!
An update: as promised, here are some GraphChi runtime results deployed on the million songs dataset and instructions how to reproduce them.