The third generation of the Unreal Engine is used for Parabellum, this engine is used for over 100 other projects. One of the reasons why Acony has chosen the Unreal Engine was that it is very easy to use it for Xbox360 Games too.
It is unique that everything gets calculated in real-time. For example, if a light source gets moved, the engine automatically calculates details like the blur-effect or the casting of shadows. The UE also provides complex Shader-Effects, Surfaces with Offset Mapping and it also loads new parts of the gameworld without disruption. The scripting Tool “Kismet” allows coders to create whole missions without typing one code-line. There are also a lot of tools for graphic- and audio-designer.
Another advantage is the great support from Epic for developers. Epic wrote an Online-Guide, called "Mastering the Unreal Engine" which contains more than 1000 sites with hints and tutorials, additional they help developers in their UDN-forums (Unreal Developer Networks).