I do agree about the time it takes to get all of your old speed back. When I learned Dvorak, it took me two years to get back to full speed. But once I was there, I was there, and with surprisingly little effort. I have a couple of victories on typing tests that I was actually trying to lose to prove it.
Nope. I still type Dvorak. One switch is enough.
I can type QWERTY, too. If I use a different machine, I'm able to switch over with surprisingly few errors. Also, strangely, I can't tell you where the keys are on a Dvorak layout without mentally typing the letter. I used to know the QWERTY layout by heart, but never reached the same level of mental mastery with Dvorak.
The only reason I suggest Colemak is that I've heard it's optimal for programming. I still don't think it's worth learning, though, if you already touch-type QWERTY.
But honestly once you get away from Qwerty there's a law of diminishing returns. The differences between non-Qwerty layouts are pretty miniscule compared to the difference between Qwerty and good layouts.