I've recently signed up. I like the idea of having my playlists available offline whilst at the gym. I think things have almost come full circle in regards to paying for music in that you pay for the convenience as opposed to the actual media.
Offline playlists are the reason I go for the paid version as well, personally I find that hugely useful for my commute each day.
I pay, because I want to contribute to the artists. I know the system isn't perfect with money going back to the them, but Id largely blame the labels for that. Two or three labels pretty much own the entire music industry even that cool little indie independent record label you love.
What Spotify need to do to stop having issues with artists is to pull their free music or at least stagger the release so the paying customers get new records earlier. My other issues with it is, when I buy a record I can have it forever if I so wish, with Spotify, I don't have any such comforts. I am merely renting my music and am at the will of artists and labels on whether I get to hear it or not.
Overall, it's the best thing to come around since piracy basically fucked up everything, though it still has a long way to go until it is as fair as it pertains to be.
The problem with doing things like staggering releases so that paid subscribers for streaming sites get this later than the album release is that you significantly reduce the benefits of the streaming model, maybe the music industry is happy to do that, thinking it will drive more album sales, personally my view is that it probably just drives more piracy.
Streaming services as it stands actually offer greater convenience and functionality than piracy for many users, the same can't really be said for physically buying or even downloading an album legally.
I agree the economics may not be amazing for artists and labels but its a lot better than having the content taken for nothing, the golden days of being able to make a good living from selling recorded content is over for a lot of acts anyway, get exposure for your music any way you can and try to make money from touring and other ancilliary revenue.