I was at first doubtful of switching from Dvorak because when I visited this forum a few months ago, I didn't see many posts from people who switched from Dvorak.  Well here's my contribution!

Over the summer, I switched to Dvorak because I heard so many good things about it and had some time on my hands.  I think I had tried switching to Colemak around week 6, but quickly switched back after a few hours of painful struggle.  After 8 weeks on Dvorak, I was at 75 wpm (I had been at 70 wpm on QWERTY after years of use), and I decided that I had enough with the copy paste shortcut positions and switched for good.  It only took me one week to get up to a decent speed of 50 wpm after switching to Colemak.  It has been 5 weeks since I switched over, and I'm now typing at a respectable 80 wpm.

But I believe that these results have to be taken with a grain of salt.  I think the brain gets better at learning new layouts the second time around, so the speeds don't necessarily mean that Colemak is faster than Dvorak.  On the contrary, it is my firm belief that Dvorak is much more comfortable for typing prose, and can ultimately achieve faster times.  There were noticeably more inward finger rolls and hand alternations in Dvorak, which made that slightly more comfortable.  However, there is more to using a keyboard than just typing prose, and for me, Colemak is my recommendation for computer use.  If you are a Dvorak user and you find yourself missing the good old days when you could ctrl+c with your left hand and click with the right hand, then I think Colemak is for you!

Another thing I want to mention is that Colemak makes typing in QWERTY a lot easier.  I remember that after I switched to Dvorak cold turkey, my QWERTY speeds completely died.  I could no longer touch-type, and when I typed, it was hunt-and-peck at 25 wpm.  Now that I use Colemak, I find that I still have to hunt-and-peck, but the hunting has gotten a lot easier because my eyes look in the right direction for letters because the layouts are so similar.  I can type in QWERTY at 40 wpm no problem without practice.  This might be something you should consider if you have to use internet cafes or use smartphones or have to use other people's machines often.