Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
Before unity I was writing an entire engine by myself, which was too complexe, too time consuming and way too difficult to pull off alone in the time I wanted to do it in.
Unity has (almost) everything I need. It has it's downsides, but It has sped up my development time 10 fold since I switched over.