Autodesk is pure and simply for character animation - not the modeling, animation and texturing you get in other 3D programs. With this tool, even those on a small budget can create impressive character animations. Be warned however that if you're not familiar with 3D modelling applications, this will really take some getting used to.
Autodesk MotionBuilder really is a dream for character animators though. You can perform just about any animation including complex reverse and mirrored effects. The developers say that it was originally developed for biped characters but this version now includes support for inverse-bending knees and elbows (such as those seen in animations of birds and dinosaurs. It's also been specifically designed for those that need extra control over characters that have more than two arms - great for making spectacular animations of four legged friends or monsters.
If you're a serious animator, you shouldn't be without this but amateurs beware - this is a tool strictly for the professionals.