I was touch-typing in QWERTY with about 80 wpm since Jan 2013. Since I'm studying computer engineering I spend a lot of time at my computer programming and I became increasingly unhappy with QWERTY's awkwardness. In Nov 2014 I decided to learn Colemak. It was definitely a good choice. This layout just makes so much more sense than QWERTY.
I switched cold turkey and practiced on average an hour a day on keybr.com (it was the holidays :P). The result: I went from 80 wpm QWERTY to 80 wpm Colemak in 3 and half weeks (both these speeds are keybr.com's average speed per day using lower case only lessons). The letter change I struggled the most with in the beginning was 's'. Although not the reason I switched, the thing that I am most grateful for Colemak is Caps Lock being Backspace, the other backspace used to hurt my pinky finger. I am faster on Colemak than I ever was on QWERTY.
Unfortunately lately I've been having to use other Windows computers at university and cannot get the portable Colemak version to work on them which is frustrating. I try to use Linux as much as possible since I have completely forgotten QWERTY and have to look at the keyboard to type.