uber moves pretty fast. I think their speed of innovation is commendable. But 17 billion USD evaluation still seem unjustified to me.
For that the valuation seems right and may even have to rise. It's all on paper. Think of Uber as a global taxi and delivery-service - basically aiming to become a new monopoly or the strongest player there. Taxi is very fragmented with money going to lots of pockets. Uber is one pocket.