Hi,
I thought I'd share my experience, now that I am using Colemak for about 7 months with a TypeMatrix keyboard.
I didn't have any pains using qwerty, and I could type reasonably fast. I was merely curious.
At first, I used a normal (now: "retarded") keyboard with Colemak.
I first started to do the lessons linked to from colemak.com for a few evenings, then using ktouch on Linux,
and finally gnu typist (at least I think that's the name of the program).
At first, I was going very, very slowly. I was trying to use colemak for typing my normal texts and program code, as I am a software developer.
My progress wasn't very fast until I started using colemak all day, accepting the fact that I would be much slower in the beginning.
I improved  a lot within something like 2 weeks.
Then I got a TypeMatrix keyboard and started using Colemak on that straight away.
The transition was again a bump in the improvement, but I got more comfortable every day.
In the beginning, I sometimes felt some pain in the left hand from typing, but that faded after I got more used to the new layout.

I found that I move my hands about much less than with qwerty, and that I can leave them resting on my wrist-rest most of the time.
In combination with the TypeMatrix keyboard (that thing is really a blessing!), for me the Colemak layout is very comfortable.

But be warned, at least in my case the transition time was hard, even though not as hard as I had expected in the beginning.

Cheers,
Christian

EDIT: Thank you for making Colemak! It's neat!