It does what it is supposed to, and the quality is pretty good when you are just running the WiiSpeak channel - but during games, the quality is awful.
I bought this to play Monster Hunter Tri online with a friend, and after months of trying, shouting at each other, repeating everything we say 10 times etc, we've just had to give up.
We both have high speed internet connections, live less than a mile apart, but the quality during a game is virtually unusable.
We've had to resort to using Skype on our laptops (running through the same internet connection, with good enough quality that we can actually understand each other.)
If you want to use it for sending voice clips or the Wii Speak channel, it's pretty good - if you want to use it during games (well, I can only speak for Monster Hunter), it's borderline broken, and not worth the effort.