I've been touch typing for about 12 years, and switched to Colemak last July because I was starting to develop symptoms of RSI after holding down a programming job for 5 years :P

Previous QWERTY speed was about 90 WPM with 98% accuracy.

I tried really hard to cold-turkey; printed out the little keyboard layout and taped it to my monitor. Sometimes in the first couple months I had to switch back just to type extended documents that were just too frustrating to backspace all the time. After one month my Colemak speed was only about 25 WPM but my accuracy was very near to 100% (a higher accuracy than I had with QWERTY)

Now 1 year later I am typing at 100 WPM with ~99% accuracy with Colemak; I never feel strain in my hands even when typing for long periods of time. I use an Ergo 4000 (it seems like a lot of others here do as well) and it's easily the best keyboard I've ever used. All in all Colemak was 100% worth it.

Funny anecdote: I took the MCAT (medical school admissions test) in January, when I had been using Colemak fairly exclusively for at least 4 or 5 months. It's a computer-based multiple choice test, but there is also an essay component. Although I didn't think about it beforehand, when I got to the testing center, I realized that the computers would be QWERTY! We were given 15 minutes to prep for the essay... I pretty much spent the entire time trying to re-learn some QWERTY! I was slow and made lots of errors, but I got through in the end. Now I occasionally refresh QWERTY to make sure I don't get stuck like that again. Although my QWERTY speed is pretty low now, if I look at the keyboard while typing QWERTY I can do 50 WPM with a moderate error rate.

The funniest part of using Colemak is when other people sit down at the computer and wonder why they have such a hard time spelling that day :D