[TheBindingVoid]

Sketch looks promising. But I would expect a couple of more features from an efficient screen design tool:

Reusable elements (FW symbols), multiple pages, something like Fireworks' "Master page", (basic) bitmap editing.

[aaronmoodie]

Sketch has multiple pages support, and symbols/reusable elements are to be included in v3

[jongold]

There are multiple pages :)

[etherealG]

is there no form of bitmap editing at all?

[jmdenis]

Very basic so far, deleting pixels by double-clicking the bitmap image and use the marquee tool + backspace.