My mouse is actually critical to how I use i3 - I've got a lenovo thinkpad usb keyboard, with the left, middle, and right mouse buttons below the spacebar and the red nub for the pointer. I use focus-follow-mouse and when I need to change my active window I just flick the mouse over to where it needs to be - it's super fast. It also works really well with laptop trackpads and an actual mouse.

I know other window managers allow you to use focus-follow-mouse, but I haven't found any that make it nearly as effective as i3.