I have to be the downer here: I strongly advise you don't get this mouse. And if you're thinking about it for gaming, absoLUTEly do not get this mouse.
It's actually a terrific mouse with one huge gaping flaw. It feels great in hand - it's small, but it fits my big hands just fine nonetheless. It tracks great in a small space, and I've noticed no visible lag. The surface texture is just right - not tacky, not slick. The mouse wheel is near perfect - spins easily, but is tight and doesn't flop about. And yes it serves as a middle mousebutton too, and is easy to click without being so easy that you click it accidentally.
Now here's the flaw: The power-saving mechanism on this things is the pits. It falls asleep after literally one second of inactivity. (I've actually timed it, I'm not exaggerating.) Not a big deal, except that when you move it to wake it up, the mouse pointer stutters and then shoots about three inches across the screen. This makes it very difficult to click on small targets like expanding directory trees and such. It falls asleep so fast that you'll find it jerking around all the time, even when you've still got your hand on it and are actively using it. Scroll down a webpage, take one second to find the "Next" link, and by the time you start moving towards it, it's fallen asleep. So it wakes up and jumps right over the "Next" link.
It really is incredibly frustrating. If it waited a reasonable length of time before nodding off, like 30 seconds (or heck, even 5 seconds!), it would be a minor quibble at best. But it's so aggressive about going to sleep - and so spastic when it wakes up - that it really interferes with normal usage. And unfortunately, there is no software or other configuration to let you increase the time.