Design - 9.4/10
I really like the Razer line of mice. They're comfortable, and they look pretty awesome. The Razer Naga MMO mouse is probably the best looking one though, because the hotkey buttons on the left side add a bit more complexity to the look. It's incredibly comfortable, more so than I expected. The Naga is designed to be comfortable for long gaming sessions, which does your hand well on a particularly long quest with your guildies.
Function - 10/10
The Razer Naga is one of the only two gaming mice I've used, but I have to say it performs incredibly well. The 12 buttons on the left side (which access the number buttons above the letters on your keyboard, which you bind your spell/skill hotkeys to) respond so quickly, it blew me away when I used them. In addition, they're easy to tell where each button is and access them, without having to look at the mouse at all.
Price - 6/10
The price for the Naga is a bit hefty, at a good $100. (The Razer store charges only $80, but shipping charges will bring it up to about the same.) But I'd say it's worth the cost for such an excellent mouse.
Overall - 9.4/10
This mouse is perfect for an MMO gamer. No more reaching across the keyboard with your left hand to reach your spells/skills, that hand can be completely reserved for movement and opening interface windows.
The only problems I have with it is that it doesn't serve you immensely well for other game genres.
Grynder
