I was taught touch typing back in 4th grade, so I'd been very fluent in Qwerty. A couple of years ago I taught myself Dvorak, and found the change to be revelatory. I ended up switching back to Qwerty because of some of the issues mentioned over and over again: shortcuts being rearranged, right pinky fatigue, but what really motivated the switch back to Qwerty was a trip to japan, and being extremely frustrated with the Japanese/Qwerty keyboard, and the inability to input Japanese on my own keyboard using Dvorak. SO I switched back. Then I figured out how to change the way Japanese was keyed in, such that I could now input Japanese using Dvorak (using kotoeri on the Macintosh). Then I decided to check out Colemak to see what the fuss was about.
Now that I've reached a certain fluency with Colemak that I don't have to think too hard about typing, I feel I can post about my experiences. I've been using Colemak for about a month and a half. When I switched, I did so cold turkey much like when I learned Dvorak. On my personal MacBook, I used the keyboard viewer so I did not have to look at my keyboard. For my work Windows box I printed out the layout and taped it to the top of the monitor (again, a strategy I used when I learned Dvorak). However, this time I used a typing program to help me learn the layout. Moreover, this time I knew how to switch the keyboard for Japanese so I didn't have to go back to qwerty when I wanted to input Japanese.

The typing progam I used was aTypeTrainer4Mac. I highly recommend it. It measures several dimensions of typing ability, but I think one of the coolest features is that it shows you all the keys and associated error rates. Interestingly, I reached a maximum of 60 WPM in a month (so far), but my accuracy has stayed constant at 95%. Unfortunately I don't have measurements of WPM or accuracy in Qwerty or Dvorak.

Now onto the hard part, the subjective review... Having learnt and achieved decent typing speed with Dvorak, Colemak didn't really blow my mind in terms of comfort. It is extremely difficult for me to tell if it's significantly more comfortable than Dvorak, only that I know both are much better than Qwerty. I've come to believe that all of those metrics that we see based on various typing measures are not as fine-grained when you map them to human experience. I'm sure that there is a good degree of correlation between subjective comfort level and those numeric scores, but when it comes to the difference between Dvorak and Colemak those differences are smallish difficult to parse when you actually sit down to type. That said, if you switch from Qwerty, Colemak offers several advantages over Dvorak.
On the subject of right-pinky fatigue, recently I was typing stuff that required a lot of brackets and braces, so that advantage over Dvorak was sort of nullified. I remember reading somewhere that Colemak also has more bottom-row use, and I do feel like my fingers generally stay on the top two rows using Dvorak to a greater extent. I just want to point out again that these are only my impressions.
It's been interesting to compare the different layouts.