The mistake, IMO, is assuming that they will be able to replicate that in other cities. In NYC you can get a taxi 90% of the time by holding your hand up in a street corner. At peak times that's more difficult, but that's also when Uber has surge pricing. They will hold their own and take over the premium market from various non-descript livery car companies - but only the premium market. They won't take over the taxi market as a whole. Much the same applies to London.
Also, much of their current success is based upon how heavily regulated and backwards taxi authorities are. For example, in NYC taxi drivers are not able to use phones or navigation aids, and hailing by smartphone is in its infancy. But that won't be the case forever.
So yes, they deserve a good valuation. But, to me, there are still a lot of questions about whether they will expand to fit this valuation.