The first pain was RSI that I got through typing a lot. I decided to try Dvorak to see if it helped.
The second pain was touch typing incredibly slowly for a few weeks whilst I learned the layout. It was a great experience, typing is a lot more comfortable, and I've never looked back.
Regarding the negatives:
You need to switch back when using other peoples’ computers
Kind of. If you type on their computer for 5 minutes, just hunt-and-peck. If you type on their computer for an hour, go to the control panel and switch their layout into Dvorak!
Nearly no availability of keyboards with dvorak layout
It doesn't matter at all. If you're using Dvorak for a sensible reason then you're touch-typing. If you're touch-typing you're not looking at the keys.
Having to relearn all your hotkeys
I can understand why Vim's hjkl would cause an annoying transition, but location independent hotkeys will cause you no problem.