I just made the switch from QWERTY to Colemak this summer. Basically I started with only using it at home doing typing tutors and using it as my system default. Once I was up to ~30-40wpm after a week or two I switched my work computer's default over too. I'm now ~10wpm faster then I was on qwerty.
I mainly program and email at work, and Colemak was my recommended keyboard by the analyzer listed in here. Loving the layout, feels easier on the hands, and a big plus for me is my most used keyboard shortcuts remain the same seeing as the bottom row is almost identical.
Negatives is I find it hard to switch back to qwerty on demand on foreign computers where colemak isn't set. Have to resort to hunt/peck, but hey at least any keyboard you come across will be labeled correctly.
I'm planning on going back to qwerty for a week sometime and then keep using it occasionally to keep in fresh to try to get around the issue of using other computers.
My recommendation? Start with it at home with the basics where productivity won't matter much, once you have the basics down where you know where each key is you just are slow/inconsistent to it switch work over. I then saw a rapid increase by using it so much. The layout is great! :) And if you might run into other computers try to keep QWERTY up with using it occasionally.
Oh, and it will piss off others who use your computer, so make sure you have separate profiles set up at home so it's not a language bar war. Wife and I used to just both use the one account.