Are there any useful smart contracts that working already? Or is this just proof of concept?
It is definitely beyond proof of concept, but its not really ready for prime time yet either. Here is a list of currently public DApps live and in development:
http://dapps.ethercasts.com/Ethereum is definitely not another shitcoin. It's purpose is not as a currency at all, it is for fuel to run apps. If Bitcoin is gold, Ethere is Oil. That said, like any commodity, it can hold value and since it is easy to trade, it can certainly be used as a currency.
Honestly though, this bubble is happening WAY, WAY before it is supposed to. The Ethereum network is in "Frontier" release which was meant to be an alpha type release. The next release is "Homestead" which was supposed to be a stability focused beta type release, but Frontier was much more stable than originally anticipated so they are added some additional features instead.
Most of the major DApps are still in development and no one serious wants to launch prior to the Homestead release.
If we are really being honest here, the primary reason why Ethereum's price has shot up is likely Bitcoin people hedging their bets due to the uncertainty around the blockside debate and developer debate. Fear over forks and a bunch of other technical stuff that they do not understand. Ethereum looks like a relatively stable coin that can be used to hedge their position.
I am long on Ethereum, but considering selling now after this pump. Its value is supposed to come from contract execution fees, not from scared Bitcoin speculators.