The reason the internet was invented. Lots and lots of music: independent artists, oldies, jazz, classical, world. Still the best deal out there. It also has a good recommending algorithm. Most importantly it has a great group of editors who spend time making lists and writing well about why they like what they like. Cons: some tracks appear, then disappear, especially if from other countries.
Adding new major labels everyday. Been a member for over 3 years and I rarely wasted a track in my allotment. Excellent for discovery. Lots of new and especially older music not available elsewhere Cons: Non-US residents don't get access to all of the tracks. This is getting more common I hear.