I have been using Sketch app since middle of 2014 I think. And it's the tool for digital user interface which I was previously using Adoble Illustrator. Illustrator is still the tool for do complex vector illustration or what not. But for doing the user interface, Sketch is ideal tool.
From Wireframe to Mockup to Production Assets
Yes. You can do it all.
For mobile user interface. To website to. Screenshots.
Many plug-in to extend what Sketch app can do.
Great companies uses this, and some tutorial and paid course teach about Sketch.
At price of $99. It's a cool price that you can use for long time and also generate some income.
Overall it's a nice and complete solution for designer for digital products.