I got mine in today and I have to say I'm not 100% happy with it.
I've tested it out with a couple games. Electroplankton, Nintendogs, WarioWare and Yoshi Touch & Go.
Nintendogs is worked great with, they head my commands better then when I used the built in mic. And when I set new commands they came out nice and clean.
Electroplankton was so-so, I was getting alot of static when my voice was recorded.
WarioWare and Yoshi Touch & Go I found that the mic was 100% useless when it was attached to my ear you had to take the mic off and put it right in front of your mouth in order for the game to pick you up.
I'll be testing it out with Metroid Prime next time I see one of my friends online.
So far I don't think its worth the money but if it works well with Metroid that might just change my opinion.