If you have snapz pro installed, the logitech drivers will not load and crash on startup.
You have to remove snapz pro (they say to reorder the extensions so the mouse driver loads first, but this did not work for me. To reorder the extensions, just name the mouse driver with an AA in front of it or something so it is first alphabetically)
I've also found an odd conflict with Logitech and BBEdit 6.x demo.
If not, and you want to install the older version:
In order to fully uninstall the new version, do a search and delete, then reinstall the older version cleanly. You should be able to get the old version from the Logitech web site.
USB Overdrive is an option, but there's the usual shareware fee and to be honest I like the Logitech drivers much better.