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.
I have been programming in Unity3d for over 5 years now, and every year they add something new, something I or other developers needed. They aren't aiming to be overly complex, they are going to functionality and usability something that a lot of game engines fail to see, and fall just short of achieving but this is something Unity has not only excelled at, but has made it their goal.
Simple but Effective, that is what Unity is.
Amazing range of platform choice to build to (all with separate licencing so you aren't paying for something you don't build for). Not to mention there is a free (slightly limited) licence for pc, mac and linux.
This engine takes pure advantage of component model and allows developers to truly design something modular, which is my personal end-goal in program design.
Every developer should at least have a look at what this engine offers to see the advantages it offers over the others.