The good stuff:Unreal let's you easily improve the aesthetics of your project. The drag and drop shader editor, the huge amounts of high-quality included content let you easily reach AAA levels of quality, even with short prototypes.
Blueprints are amazing for quickly prototyping concepts! Beware: any non-trivial math expressions will be a mess of wires, but thankfully there's a handy math expression node to help you avoid turning your code into a spaghetti mess.
The community has a lot of experienced developers, which makes useful content and non-trivial tutorials easier to find.
The bad stuff:Don't get me wrong, Unity is a bit of a resource hog sometimes. But Unreal seems to be an expert at making even our reasonable Gaming PCs suffer. Compile times will be measured in minutes for anything but the smallest projects.
The way Unreal handles project files is simply not as robust as Unity's. Every time you move, a file a redirector is created. You can't just drop a png file and use it as an asset, there's always going to be some sort of pipeline to get assets into your game.
C# is gone. You will be using C++. This can be a curse or a blessing but overall C++ is not as great for quickly prototyping.
It's not very stable. Seriously, it's not. The update to 4.9 broke transparent sprites on the UI. This is not a trivial and easily-ignored bug.
The huge ingress of novice game developers has made communication with the Epic team and experienced members slightly harder due to a sea of (mostly) trivial questions.