Haven't got experience of other streaming services but I think Spotify is great, works really well and basically has all the music in the world as far as I can tell (pop/rock etc, no idea about classical). Don't think there are many artists who don't have their music on now and most the major hold outs are on e.g Neil Young, Prince. App is pretty decent, download and offline functionality works well, music quality is adjustable depending on whether data/bandwidth is an issue and seems good to me though I'm not a hi-fi buff.
In terms of prices I think its great value , £10 a month to access all the music I could ever want - certainly compares very well to things like Netflix is terms of value delivered for money spent.