I bought one last week and imho it rocks!
I use it to controll the main volume (increase, decrease, mute toggle)
Get new mail in mail.app
Back, Forward, Home and scrolling up/down through webpages
Play, Pause, Next/Previous song and Volume
Havent tried gaming yet ;-)
I paid 85 euro for it (yes hardware is horribly expensive in Holland) but still think it's worth it.
Cheers