Though small room temp ones don't exist yet, they likely will some day.
As to why they are a 'big thing': Imagine a large GPU cluster turned down to the size of a laptop and running off a solar cell. They'd revolutionize electronics in other ways too. But the main use we can see now is that they allow for great amounts of computation for very cheap energy budgets. Stuff that takes coal mines today could be done with what it takes to feed a cat.
