If you are in UI Design, there is no better tool than this. There are many mockup apps but only very few that lets you really design the GUI and present it to your customers. It is much more than simply structure your layout and place some premade components on the art board. It works more the way Interface Builder in Xcode works with the possibility to fully design your components instead of just using the included ones by Apple.
Antetype can't be compared to vector applications like Sketch and it also can't replace it. Vector design apps lets you draw individual components, where Antetype is very basic. But to build your GUI Antetype excels. The auto layout mode is simply genius and lets you just drop components in the layout and they end up just where they have to be.
It is hard to describe what makes this app so special, so I recommend watching the screencasts on their site.
Yes, the price is a little bit high but if you are a student or teacher they have an educational discount of about 90%!
This looks like an incredible piece of software. I wish it was just a little bit more affordable. I do understand that it took resources, time and the like to produce, though.
It takes me significantly longer to prototype an idea than I would like. The goal of prototyping in my opinion is to go from idea to validation as fast as possible. That means a lot of what you do will get scrapped and you want to get through as many ideas as possible. I didn't feel this way with Antetype.