Meng To’s review of Sketch:
Photoshop has a big legacy and I can sense the unwillingness to make a switch and say goodbye to years of building libraries. But if you can overcome that, not only will you improve your overall design process, but you will be ready for the future as all your new designs will be completely vector-based, flexible and resolution-independent. You will design faster.
The good news is that Sketch is very similar to Photoshop and has more or less the same features that you are familiar with including keyboard shortcuts, layer blending, styles, blur, noise, patterns, etc, except it won’t have all the filters and photo editing capabilities that you probably don’t need as a user interface designer. It’s not a watered down Photoshop, it’s a robust design tool that has completely adapted to today’s design standards.
Update: Originally I linked to Meng To’s article stating it was a review of Sketch 3, but his review is over a year old. It’s still relevant, just not recent. My apologies.