Reviewed by William Savona (Currently owns)
This mouse had a lot to live up to. I owned its predecessor, the Logitech MX Revolution, and loved it. I only replaced it with the Performance MX because the rechargeable battery of the older model was on its last cycle and wasn't user replaceable. With the removal of the thumb wheel and automatic scroll wheel gear adjustment system I had doubts that Logitech had actually improved rather than deprecated, as they often do, a great mouse for people who spend all day using one. After using for just one day I knew I had somehow found a superior mouse to the Revolution MX I was reluctantly replacing.
I quickly realized that the old automatic way of changing between a ratcheting scroll movement and a smooth scroll movement was more of a practice in patience than a useful feature. With Performance MX there is a selector button below the scroll wheel that allows you to toggle between these two modes. I find now that I just leave it on the racketed movement style 99% of the time, using the smooth movement for time when I need more precision over my scrolling.
Speaking of scrolling, it's important to note that Logitech still insists on making software drivers for their mice that overrides Mac OS X's stellar scrolling algorithms causing erratic behavior in programs that are likely to have very long lists (iTunes, iPhoto, etc.) I found a program that fixes this issue while retaining all the programability of the mouse called SteerMouse.
What I think is the biggest improvement on the Performance MX mouse is the change to USB for charging instead of a cradle that renders the mouse useless while charging. The battery doesn't last as long now that it's a user replaceable AA rechargeable but that I can use the mouse while it's charging and replace the battery when it gets older are huge wins for me.