I learned QWERTY up to 70WPM in high school with a very good typing course. Didn't have problems develop until later (I think it was a neck injury that set things off).

I learned Dvorak in 2005 and had been using it for about 2.5 years. Got up to around 50WPM, and several months in I got my QWERTY back and could switch between layouts fairly seamlessly with touch typing. I learned about Colemak a while into that, and ironically one of my reasons for staying with Dvorak was the same reason that most people stick with QWERTY - many of the public computers I used could be easily switched to Dvorak.

About 6 months ago my laptop broke, and I spent several months on public computers - most of which ended up only having QWERTY (the Dvorak enabled ones had seemed to have disappeared), and I lost my Dvorak skills. The bit about Colemak that appealed to me was that the pinkies didn't have to stretch up much, which was an issue in Dvorak.

A week ago I started Colemak. I switched the keys on my Vaio, the F and J keys were attached differently so that was something of a problem, and I switched them back after 2 days, so on my keyboard N and F are switched and J and T are switched. I figure it's easy enough to remember that. Two days in I was using Colemak for regular typing, although I didn't like long sentences. About 5 days in I got comfortable touch typing normally and posting on forums and chatting on IM without too much frustration. I didn't do much typing excersises. I got as far as R and O before just going into straight typing.

So far some things I've noticed. The S key feels in the wrong spot. My most common mistake is R instead of S. My second most common mistake is using the right finger and place but the wrong hand. I think this is fairly normal, although I think Dvorak bypasses this as you know all the vowels are with one hand. I also think my S R confusion is carrying over to my right hand as I'm getting a lot more I E mistakes than I think I should.

Something that strikes me about the typing courses is that they're all for letters - there's nothing training me to use the caps as backspace, so I'm having to really consciously think about it - it's definitely good as it takes some right wrist strain away. What I really appreciate compared to Dvorak is the placement of the brackets and - = as that was a seriously confusing issue for me for a long time (I think due to Dvorak also only having letter training courses).