There's no real evidence that there is a profitable business model for streaming music except insofar as subsidies from other parts of the business make sense overall. Just because a lot of people like something doesn't mean that there is a path to making money with it.

I don't have strong feelings about what's good and bad about their UI, etc. Their main failing is that they're probably not going to exist much longer unless Apple, Google, or someone along those lines snaps them up at fire sale prices and is OK with losing money on them.