Still have to try it 'seriously' to have a proper opinion about it.
Update 3 (December 2016): JustInMind Prototyper also seems interesting for Windows, even if it doesn't handle greatly HiDPI and doesn't seem as intuitive as Sketch. (not having an object navigator kills it for me for now)
Seems it has quite a long way to go to have all the Sketch functionality though, very very beta. Update 2 (December 2016): It's now available for Windows 10 Anniversary Update as well. Sadly only for Mac now, but it'll be available for Windows in 'late 2016' according to Adobe. Update (February 2016): you can now try the public preview of Experience Design CC (previously known as Project Comet). We'll see how it goes, but right now I use Sketch on a El Capitán virtual machine with VMWare and it works pretty well! It seems that they are now creating a copy 'heavily inspired by Sketch' app: Project Comet. Fireworks is probably the closest, and it's no longer supported by Adobe. And no, Illustrator and such are NOT Sketch alternatives. After trying a bunch of Windows applications, I haven't found anything equivalent to Sketch for Windows.
