I discovered Sketch about a year ago, and I should say it keeps getting better comparing to Illustrator. There are still some operations I can't do in Sketch, but it is the fastest and the lightest app to design UI such as http://dribbble.com/shots/1111784-iOS-7-icons-redesign/attac.... It took me few hours to redesign a whole iOS7 icon set in Sketch. Last months I use Illustrator to convert files to SVG to work on them in Sketch. The sad thing is that it works on Mac only and is not adopted by the designer community yet.