A co-worker of mine uses one of the stranger keyboards I've seen, a Kinesis Advantage.
He picked it up his after a bout with tendinitis and was sold on it. He was kind enough to let me borrow his spare for about a week so I could try it out. It's been an interesting week. :)
The Advantage differs from conventional keyboards in a number of ways, the ones I think most relevant are:
C key is directly below D, not below and to the right. Moving your fingers from their keys on the home row to the corresponding keys above and below is a much more natural movement.
Backspace, Delete, Home, End, and Control and Alt keys are positioned within reach of your left thumb, your right works Space, Enter, Page Up and Down, in addition to another Control key, and a Windows key (which I remap to Alt). This really makes sense if you think about, why waste two perfectly good fingers on the same key, when you could put them to use and eliminate all of that reaching.
I'm not going to lie though, it does take some getting used to. The biggest problem I had was Space vs. Backspace, which are the right-most thumb key, and left-most thumb key respectively. Prior to all of this I heavily favored my left thumb for striking the space bar, and muscle memory is a bitch when it causes you to Backspace when you meant Space.
Other points of frustration were the tilde/back-tick key (located bottom-left instead of top-left), and the left and right bracket/brace keys (located bottom-right). These keys are used a lot in a shell or when coding, which probably made the pain even more pronounced for someone like me.
I managed to force myself to use nothing else for several days, at which point I felt I was doing quite well. I still had the occasional problem here and there, but it seemed like I was well on my way to normalcy. Then I tried using the built-in keyboard on my laptop. Wow. Epic fail. It took a few more days and plenty of patience before I was able to move back and forth (and truth be told it's still a little awkward).
So was it worth it? Yeah, I think so. I've had RSI troubles of my own and a week of typing on this keyboard has felt pretty good. I've ordered one of my own to use at work, and I'll probably grab a second one for home.