I never used caps lock. So I started it to map it as CTRL, but only benefited from it in a terminal or SSH session: I'm using a Mac, which has all important shortcuts mapped to the Command key. So I thought about mapping it to the Command key, but I use my thumb for pressing that key and that doesn't cause any strain (as opposed to using my pinky for CTRL).
So, after test-driving Colemak for quite some while I have it now mapped to backspace, which is wonderful. You don't have to lift your hand anymore from the keyboard when making a mistake.