This must be one of the best mice around. I’ve now had two. The first was serial/PS2, and was returned under warranty after developing a fault in the left button (hopefully a one-off). The replacement is USB/PS2.
The mouse has three true buttons, left, right and one for the thumb, and also a wheel which includes a button. With Logitech’s software, all four buttons are easily configurable. Mine are set left and right as usual, wheel for auto-scroll and thumb for double click.
The scroll wheel is comfortable to use and ridged for good grip. The number of lines scrolled per movement is also configurable, and scrolling works smoothly in 99% of applications.
The double click option for the thumb button is absolutely brilliant.
The main drawback: Do NOT buy this mouse if you are left-handed. However, for right-handers the ergonomic design is excellent.
The software is easy to install, and expands the mouse section in Control Panel. As usual, get the latest software from the web site.