Usage is like most tiling WMs, a little odd if you've never used one, but I quickly found myself addicted. Basically, when you open a window it opens full screen. Open a second, and it divides the screen exactly in half. You can then use keyboard shortcuts to move them around and resize them.
It's easy enough to set up and i3 is well documented. I have a minimal install so I can't speak for more complex setups.
I came from xmonad which I found really hard to configure... This is all plain text and lots of what I wanted comes out the box anyway.
Check out their official site and the Arch wiki - there's info on shortcuts and setup there.
If you have any questions fire away.