i3 is really nice, it looks good with the useless-gaps patch too. But it hardly has the most amount of visual customization, rather the contrary IMO. awesomewm, herbstluftwm, and others offer way more to tinker with. Its manual tiling and modal keybindings is what makes it nice IMO.
Recently I switched from i3 to bspwm, I really love the approach bspwm takes. Definitely the best designed window manager I've used.