I've been using colemak for a week now. I had been typing on qwerty for 5-6 years and could get to 50 wpm on that layout.
I have been keeping a log of the highest speed I can go on colemak. On my first day, I could only type at 8 wpm. But the speed increased to 12 to 18 to 22 and currently at a speed of 25, half the speed on qwerty. I've been using hi-games's typing test. I'm hoping to catch up to my qwerty typing within the coming weeks and months.
One thing bothered me. On the colemak keyboard, some of the keys were placed on the US layout. But after using MS Keyboard Layout creator, I set the keys to the UK layout.