As I mentioned, I chose Razer because the brand appealed to me as well as fulfilled my requirements. Many hardcore gamers will look at features first, and consider the manufacturer secondary. If a generic, unknown brand of mouse has the necessary features -- buttons, DPI, response time, etc. -- there's little reason not to use it. It'll be considerably cheaper, as well. Branded mice such as Razer, Steelseries, or Logitech might be more reliable, however, and have some form of warranty or support.
That said, it takes a bit of work to properly understand all the features of gaming mice. We've talked about the basic one, which is form factor. A good mouse has to feel comfortable in your hand. I like the matte finish of my Lachesis as opposed to the smooth, glossy finish of other mice, for example. Other gaming mice have rubber or rubber-like portions for better grip, etc., and these all count towards the tactile features of a mouse.
CPI or DPI refer to Counts Per Inch or Dots Per Inch, which indicate how fast a mouse can move, or how many steps a mouse pointer can move when you move it an inch. The higher the number, the faster a mouse is capable of moving, which is excellent for high sensitivity gamers. Higher DPI mice are also more accurate when targeting a particular point in the screen.
A mouse's DPI count is also going to be more or less palpable depending on your screen resolution. In smaller screens or if you play in Windowed mode, mouse DPI won't be as apparent as you have to move across fewer pixels. In higher resolutions, a faster mouse will allow you to move your pointer, well, faster. The kicker for a World of Warcraft PvP player is that moving with the mouse -- pressing both mouse buttons -- will make the cursor disappear and render it incapable of actually pointing at anything.
On the other hand, when you do use the mouse to target things in-game, such as when using mouseover macros, higher speed or DPI mice come in handy. Mouseover macros allow players to cast spells on targets without actually having to select them -- simply having the mouse pointer hover over legitimate targets will allow players to cast the spell, thus eliminating the split seconds wasted on selection.
That said, high DPI isn't particularly important in WoW PvP. Between your system's software controls, the mouse settings, and mouse sensitivity, most mice will perform well enough for your gaming needs. Some mice even offer on-the-fly sensitivity adjustment which enable players to adjust the mouse speed while playing the game without having to access a special control panel. This is largely useless in WoW PvP because of one basic thing: World of Warcraft is an auto-target game.
You don't have to target enemies with a cursor the way you would on an FPS. That's the easy part. PvP relies on unleashing the proper spells proactively and reactively, so having easy access to all your abilities helps set you apart from the competition. This is where multi-button mice come in.
Another key stat you might hear among some gamers is APM, or Actions Per Minute. This refers to the number of actions or commands a player can send to units within an RTS. The average professional RTS gamer can execute well over 300 actions per minute, and a few exceed 400. Beginners average around 100 APM. In contrast, World of Warcraft PvP will click on their mouse maybe 50 times, tops, because most abilities activate a 1.5 second Global Cooldown, locking you out of spells and rendering click-spamming largely useless. This means technologies like Razer's HyperResponse buttons are cool but unnecessary. As long as your mouse can accurately register a click roughly every second, you'll be fine.
The bottom line
Ultimately, World of Warcraft isn't a game that demands much out of a mouse. Arguably the most useful feature of a gaming mouse for WoW would be programmable macros, although with the proper AddOns even those can be done away with. As long as your system registers all of your mouse buttons -- the Steelseries WoW mouse has fifteen! -- Bartender or Bindpad can take care of the rest. Tomorrow we'll sneak a peek at gaming surfaces and keyboards.
Previously on gearing up for PvP: