It has its quirks and even some annoying bugs but where it excells its IMO way better than what any competing/proprietary design tool can do (vector and bitmap exporting, vector+bitmap combined layering, shape/colors/text layout, PDF editing/creating, vector pen tool, etc). I use it to create UI for games & apps and more generally to build sprawling UX scenarios and concept flowcharts.
In more recent builds its performance has become quite good which was a problem it had before. Granted, lots of room for improvement still particularly wish it had more natural flowcharting capability.