I started out with a "guest" account (gift of a UK Mudcatter) before Spotify became generally available in the USA. No ads, just music. It was too good to be true. Shortly after Spotify came to the US, I lost my "special" status, and I started getting ads. I could listen to about two or three songs before getting an ad, and that was fine for music research. Then they integrated my MP3 collection with what they had available online, and I found I really liked Spotify. So, I broke down and subscribed for five bucks a month, and I don't regret it. It's a great way to have access to a huge amount of commercial-free music.
I'm glad I have my own collection to supplement it, but Spotify does have a lot of folk music - almost everything from the Smithsonian-Folkways and Topic labels, for example.

-Joe-