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.
Unity is a really great game engine. the editor is customizable with it's own scripting api. the pipeline is customizable too.
all features are easy to use and the engine can make small indie teams powerful groups to make huge games.
there are great add ons for most missing features and it's becomming as popular as techs like flash.
unity's asset store will allow developers to cooperate and use each others work with great prices.
there is not any other single engine with this price range with these features. if comparisons were allowed and tolerated, i could post advantages of it against all other engines this side of the market (i mean less than 500K dollars and without royaltees)