I've never fallen in love with an app so fast, and so hard, as I fell in love with Sketch. And it's because I fell in love with Keynote before it, and Photoshop a long time before that.
Each of those applications contained one main feature that changed the game for me. From Photoshop, it was the graphical display of layers with controls available at each layer. And from Keynote, it was the ability to simply and easily select layers, objects, and groups by simply clicking on them in the visual display - a feature is now seems so obvious that its absence in PowerPoint makes that application an absolute terror to use.
Both of these features come standard in Sketch. Still better news is that a whole bunch of features found in other applications have been left out of Sketch on purpose, leaving the interface uncluttered and response times fast.
Aside from the two big ones, Sketch offers a number of other benefits:
The application supports artboards that are easily duplicable in the user interface. You can copy, drag, and replicate a new version of the screen you were just working on in only a few clicks.
Sketch is also 100% vector, which makes scaling up and down supremely easy, and is especially useful when designing text-heavy interfaces.
Lots of slick export tools, built right into the interface, make it easy as pie to get your images out of Sketch and into raster images or PDFs that others can digest.
The Sketch community is still small, but there are a number of plugins available for the app that allow you to extend its core functionality. By far the most useful of these I've found is this symbols plugin, which creates Fireworks-esque symbol capabilities in Sketch. Boom.
Sketch has a companion iOS app called Sketch Mirror that allows you to see a preview of your design on your device in real time. No more exporting, mailing and opening on the device. If you're an iOS designer, this is an incredible feature.
As with any new product, there are bound to be some drawbacks. Here are the most obvious ones I've discovered with Sketch so far:
Relative to the big boys in the space, Sketch has small adoption, which means that you'll likely be the only person in your office using it.
At larger file sizes, I've started to notice some lag in the rendering behind my documents. This can be frustrating when you're trying to move around a document with a lot of artboards, each being used to display a specific screen in your application. It's solvable by breaking your screens out into different documents, but hopefully is something the team at Sketch will get better at over time.
You'll encounter the occasional weirdness in Sketch, such as rulers that don't always move how you want them to or phantom layers that don't seem to move where you put them. These are to be expected from an app that's still in heavy development, and they're usually solved by a simple restart. It's really not that big of a deal, and if you have a specific problem I've found the support team to be very responsive.
Overall, my experience with Sketch has been super positive, and I'm not planning on going back to either Keynote or Photoshop for my UI needs anytime soon. The application has enabled me to work quicker and with more precision, and I hope to convert more of my team in the future so we can all share in the glory of 100% vector, 100% easy designs.