We did have Muzzy, which DS used when he was 3-6 years old or so. Mostly he loved to listen to the tapes as he was going to sleep at night--he found them very comforting. He also liked the computer games that came with it. I don't remember that he spoke much, but I felt like he absorbed a lot of the sense of the language through all that listening.
When we took him to Honduras the first time when he was 7, it was like a light went on for him. He understood that Spanish wasn't just a silly game, but the way people communicated. After that we haven't had any trouble getting him to speak. But I do think that Muzzy laid the ground work for his later learning. I doubt very much if it would work on its own, though; you would want to have someone for your child to speak with, at least eventually.
