Does or has anyone have or tried MMD (MikuMikuDance) program?

If so, how does it work? I am thinking I may want to try it out someday.

I have seen so many funny and amazing videos based on the Vocaloid characters doing different things and I would like to try it out sometime. But I need some insights like where do I find it? Can it be downloaded? And what features does it have?

If you have no answers for me, it's okay. I was just wondering about this program because after watching some MMD videos on Youtube I may want to take a crack at it someday and just want guidance.