I have been using colemak for about a month now, having switched from somewhere between 50 and 60 wpm on qwerty, and am up to 46 wpm with an error rate of 6 on keybr.com's test today. I have spent a total of 13 hours or so on that site over the last month (it totalizes for you), and probably as much again on other typing tutors.
Typing was a real mission the first two weeks, after which I could start functioning semi-normally again (say 25 wpm, with lots of concentration - it was very difficult typing emails and trying to think of what I wanted to type and how to type it at the same time. Thus typing tests which supply the text were much much easier to type that emails etc.).
I switched cold turkey, but did relabel my keyboard, since I do a lot of one handed typing in applications such as Autocad. The switch became much easier once I put the labels on. I don't think that the argument that it allows you to cheat by looking down is a very strong one, especially if you have touch typed before - just don't look down! I used a brother label printer to print labels for the keys. With rounded corners they don't peal.
I still have a bunch of habits that bug me a little, especially combinations that I used to type with one hand out of touch typing position - say Alt F,S to save, or cd (as in change directory) - old habits. But is is getting easier every day, and my speed keeps improving by 1 or 2 wpm per day on keybr.com.
All in all I am very happy with my decision to switch - the layout is starting to feel really smooth and easy now (much better that qwerty), and I think there is still a lot of speed as my fingers learn to quickly do common combinations of letters. The rolls took a long time to become habit, but are really starting to kick in now.
I ended up disabling qwerty completely in the control panel, setting colemak as the only layout, because otherwise every now and again an app would start up in the wrong layout.
My feeling would be that it would be a LOT more difficult to learn colemak while maintaining qwerty than just switching cold turkey.