In January I moved from Qwerty to Colemak, and also from a staggered to a columnar keyboard layout (on a Kinesis keyboard[1]). My Qwerty speed was only 75 wpm and my accuracy was terrible. Also I couldn't touch type - I never learnt to type properly with Qwerty; I just started typing when I was young and developed lots of bad habits. By the time someone tried to teach me how to type properly my habits were too ingrained to change. (Or maybe I was too stubborn -- I also have a strange way of holding a pen.)
Anyway, I decided to change to Colemak, and to learn to touch type properly. I also liked the idea of a columnar layout so I changed to a Kinesis. What's interesting is that because of the physically different keyboards, I actually cannot type in Qwerty on my Kinesis, and I also cannot type in Colemak on my laptop keyboard. This means I have maintained my Qwerty skills.
My current maximum speed is roughly 70 wpm in Colemak, so I am almost at my Qwerty speed after 6 months, although now I can touch type so it's more useful. When I type with Qwerty now, it feels like my hands are flying all over the place. I also feel like I have a lot of room to get faster than I currently am with Colemak.
So even though I have maintained my Qwerty skills, the only reason I need them is because I can't get a laptop with a columnar keyboard. I may end up just biting the bullet and switching to Colemak on that laptop keyboard anyway, though missing the thumb cluster is killer.
One other interesting thing I learnt is that using Colemak on a smartphone completely sucks. Having commonly used keys on the home row on a small touchscreen means that it's much more error prone, as you're hitting the same area on the screen consecutively.