Negative side, it's bound to Mt. Gox so you can't use it as a normal YK on any other site.
Did you try it on http://demo.yubico.com/php-yubico/one_factor.php ?
It is pitty if the key cannot be used outside MtGox (yes, I've read their EULA
AFAIK yubikey has 2 slots for secret key, they can be switched by long tap. I wonder why they removed Yubiko key instead of using the second slot. If they would leave Yubico's secret key then the key could be used on other sites for authentication...I was under the impression that MtGox used both keys.
Download the personalization tool and take a peek at it:
http://www.yubico.com/personalization-tool
I have two Yubikeys on the way for password database use (ie: Passpack.com, Lastpass.com, 1Password, etc) and have been reading up on them. Going to try to get the wife to use one... *crosses fingers*