The only negative I can say about this mouse, is that I have a different one at work and it is easy to get used to more features rather than less.
REPLACEMENT TO A PREVIOUS MICROSOFT MOUSE It was not a real stretch for me to switch to this mouse as I previously owned the Microsoft Intellimouse.  However, this mouse is bigger with two extra buttons. I was hesistant to change, but the old mouse's roller had gotten real difficult to roll (Cleaning and disassembly could not fix) and once you get used to that roller in between the left and right mouse button you are hooked.  The intellimouse is also a clearly right handed mouse, so a similar design attracted me to this mouse.  Also to be honest the solid microsoft name behind it helped too.
EXTRA BUTTONS A REAL PLUS The two extra buttons on the left of the mouse, handy to your thumb are initially programmed as the forward and back selections on a browser.  If you do much surfing and find you use these buttons alot, you can get used to this quick.  I never realized how much I missed these buttons until I found myself searching for them at work.  You can program these buttons to do different things if you want to, but the default was fine with me.  In fact that is true with all the buttons.
SIZE IS BIGGER THAN OTHER MICE This is the biggest mouse I've ever owned.  You might want to try it out before you buy.  It was not a detractor for me because I have fairly large hands.  For that reason I may notice it more, as rarely does a mouse take the total length of my hand from the heel of my palm to my finger tip.  Ironically, what I read in other reviews is that smaller handed people had no problems, but larger handed people really noticed but did not mind it.  Maybe they trully found a one size fits all.
SOFTWARE INSTALLS NICELY Not surprisingly the software installed smoothly.  Ironically, I accidentally deleted it in a restore to my machine that I had to do for other software and the mouse works fine without it too.  I suspect all the software adds is the customizability functions.
GOING TO OPTICAL NOT A BIG CHANGE As my previous mouse was not having ball problems, I didn't really notice a big switch when I went to optical.  I had always been pretty clean about the mouse, cleaning it every so often and keeping up to date with good mousepads.  However, if you had used older optical mice prior to this generation, I need to warn you.  These mice work seemlessly without the errors and fussyness of older optical mice.  All I did was literally unplug my old mouse, and pop in this one, keeping my nice mouse pad with the gel wrist rest.  It works the same. I used the PS2 outlet and not the USB Port as mine are all full with printers, camera connections and CD burners.
LIGHT ON BASE OF MOUSE NOT BLINDING If you are a gamer and like to be in a darkened room this optical mouse will not blind you.  It has a light on its base that is always on, but since it is red (like the old darkroom lights used to be) it will not detract from your vision if you play in a dark room.  It is easy to ignore.  However, it does make the mouse easy to find.
All in all a decent mouse.  Only time will tell how durable this will be relative to the old style ones.
The only negative I can say about this mouse, is that I have a different one at work and it is easy to get used to more features rather than less. REPLACEMENT TO A PREVIOUS MICROSOFT MOUSE It was not a real stretch for me to switch to this mouse as I previously owned the Microsoft Intellimouse. However, this mouse is bigger with two extra buttons. I was hesistant to change, but the old mouse's roller had gotten real difficult to roll (Cleaning and disassembly could not fix) and once you get used to that roller in between the left and right mouse button you are hooked. The intellimouse is also a clearly right handed mouse, so a similar design attracted me to this mouse. Also to be honest the solid microsoft name behind it helped too. EXTRA BUTTONS A REAL PLUS The two extra buttons on the left of the mouse, handy to your thumb are initially programmed as the forward and back selections on a browser. If you do much surfing and find you use these buttons alot, you can get used to this quick. I never realized how much I missed these buttons until I found myself searching for them at work. You can program these buttons to do different things if you want to, but the default was fine with me. In fact that is true with all the buttons. SIZE IS BIGGER THAN OTHER MICE This is the biggest mouse I've ever owned. You might want to try it out before you buy. It was not a detractor for me because I have fairly large hands. For that reason I may notice it more, as rarely does a mouse take the total length of my hand from the heel of my palm to my finger tip. Ironically, what I read in other reviews is that smaller handed people had no problems, but larger handed people really noticed but did not mind it. Maybe they trully found a one size fits all. SOFTWARE INSTALLS NICELY Not surprisingly the software installed smoothly. Ironically, I accidentally deleted it in a restore to my machine that I had to do for other software and the mouse works fine without it too. I suspect all the software adds is the customizability functions. GOING TO OPTICAL NOT A BIG CHANGE As my previous mouse was not having ball problems, I didn't really notice a big switch when I went to optical. I had always been pretty clean about the mouse, cleaning it every so often and keeping up to date with good mousepads. However, if you had used older optical mice prior to this generation, I need to warn you. These mice work seemlessly without the errors and fussyness of older optical mice. All I did was literally unplug my old mouse, and pop in this one, keeping my nice mouse pad with the gel wrist rest. It works the same. I used the PS2 outlet and not the USB Port as mine are all full with printers, camera connections and CD burners. LIGHT ON BASE OF MOUSE NOT BLINDING If you are a gamer and like to be in a darkened room this optical mouse will not blind you. It has a light on its base that is always on, but since it is red (like the old darkroom lights used to be) it will not detract from your vision if you play in a dark room. It is easy to ignore. However, it does make the mouse easy to find. All in all a decent mouse. Only time will tell how durable this will be relative to the old style ones.