RE: Just bought Logitech MX 620, doesn't work
Some Logitech mice do not have any support "on the Mac side".
My MX-610 is one of these. Yet, when I connected it to the Mac (in this case a g4 running 10.3.9), it was recognized immediately, and several (but not all) of the buttons worked without any setup at all.
Don't worry about Logitech not supporting one of their products with their own software. Get either "USB Overdrive" or "Steermouse" and be done with it. I found that Steermouse supports a couple of buttons that even USB Overdrive doesn't, BUT, USB Overdrive seems easier to set up.
- John