Are keyboard shortcuts really more efficient than a mouse?

As a Mac user, there is one very specific admonition from Windows and Linux users I hear frequently; too much mouse! But I'm not just a Mac peep, I'm also a Windows and Linux guy. I started as an Tandy Radio Shack kid, became an Apple ][ kid, then a PC teen, and ultimately a Mac jerk (read: young adult.)  In the mid-90's, when the public internet was spun up, I was still a Mac guy sporting a PowerBook 145b emitting a veritable thatch of SCSI peripherals. But the story gets even more complicated.