I love the simplicity of Sketch and how easy it is to learn. I love the community behind it and all the support and plugins.
I can't think if what i don't like much about the tool in all honesty. Maybe if it had a bit more options and tools like illustrator, but then again, it might make it more bulky and lose it's slick UI.
It might seem a bit complicated at first, or maybe a little underwhelming depending on your background or what tools you've previously worked with, but give this tool a chance. Once you get the hang of it, the rest is a breeze. It makes your workflow so much faster and it's Invision integration is second to none. Granted there are third party plugins you can install to do a whole platter of things, especially Craft. This plugin allows your Sketch workflow and Invision to work seamlessly together. The way the art boards work is amazing, especially depending on the type of device you're working on, Sketch gives you starting dimension and icons, and components. Lastly, do not let the price hinder you from trying the product. More and more companies are catching on, and soon it might very well become THE "industry standard".
Sketch allows for quick prototyping so it helps save time and allows for less resources to be used. It's better to work in Sketch and run usability tests, than to code and then go backwards.