At 1440p at stock clocks it'll beat a 780GTX for $100 less. That situation only improves if you go to 1600p.
If you OC both the 780GTX gets a negligible lead at 1440p and probably gets tied at 1600p. For $100 less still.
Its not a perfect card by any means but to say you need a 4K monitor to make the R9 290X worthwhile is silly.
I'm actually looking at it from the European point of view: I can find this card on sale for 520 in Italy:
http://hexus.net/tech/reviews/graphics/59777-gainward-geforce-gtx-780-phantom-glh/
But the 290x I'm being told will start at 550, which means that the 290x with stock cooler is MORE expensive than the superclocked gtx780 with custom cooler, at least in Europe.