According to the principles of capitalism, a company or a product is worth what people are willing to pay or invest. Uber is the most highly valued tech start up in the nation. If investors are willing to put in so much money, then there is no question that the company is actually worth that much.
Sure, it's undeniable that Uber and its ilk of tech company have profoundly shifted the landscape of the respective economic sectors in which they operate. Pay-per-ride transportation will never again be the same thanks to Uber's influence. But to claim a valuation of $40 billion for the company is simply preposterous. I mean, what's their annual revenue stream look like? It can't be anywhere close to that.
Uber is an amazing company. It lets average people make money giving rides, while still giving the customer a guarantee that the driver will be reliable. Uber often costs less money than a taxi, but is just as good. Uber will probably continue to grow exponentially until it replaces conventional taxi companies.
While Uber did set the groundwork for a new business market in transportation, it has been making seriously bad attempts in their public image. Not only do they devalue their existing cabbies by increasingly lowering their hourly rates, they treat their employees terribly. Their employees aren't the only ones to suffer, as their CEO has been caught trying to manipulate journalists by uncovering embarassing or otherwise material from their backgrounds. Now, Uber is endanger of losing representation in Las Vegas and possibly more cities to follow. If Uber keeps on this track, they won't even be a company soon enough.