Simply put, it's great.

Very flexible and fast enough. All the math is done on the processor though, so theres room to speed it up if need be.

Cal3d has been a great help to me from learning simple skeletal animation, to designing my more advanced character animation system. The Mixer concept for blending weighted animations over each other is excellent.

I haven't used cal3d in anything proper, just done some tests and read their code. There are commercial games using it though from what I remember.