I often forget about nebula. I really do like their model. Personally, I think their biggest problem is lacking a free trial. They're curating, so I expect they'll have some pretty solid content. But every time I consider them I go and browse their catalog and I don't recognize most of it, and that what I do recognize is marginally interesting to me, but not worthy of payment.
I suspect their closed model is because it's very expensive to host that data, and they want to make sure that whoever they put up there is worth spending the money on. Return on investment, yada, yada.