The Half-QWERTY is a really interesting way to type. It's sort of like writing in cursive backwards. It's really not as hard as you might think. You really just have to control your impulse to over think what you're doing and let your muscle memory take over.
I seriously considered one of these a couple of years ago when my left wrist was acting up in a big way, but I've never actually put my hands on a real one. I went as far as to put some real practice time into the typing demo that they have online that Guardian refered to and I agree with him that it's amazingly easy to pick up for touch typists. For people who can't touch type 100%, it might not be so easy though.
What makes it interesting and easy to pick up is that it relies on the concept that you intrinsically have the muscle memory to execute left hand typing movements with your right hand and vice versa, and a huge part of the difficulty comes from thinking too hard. This is why it might not work for people who aren't pure touch typists. It's all about motor memory. After you get comfortable with it, you just have to let yourself go and you can probably achieve anywhere from 25% to 50% of your usual typing speed.
I ultimately opted not to try one because I was expecting my wrist problems to be temporary and as bad as it was at the time, it was indeed temporary. A combination of speech recognition, mouse shortcuts, and regular one handed typing was enough to get me through. If I permanently lost function in one hand, I'd get one of these in a heartbeat.
They used to also have a much cheaper version of the Half-QWERTY called the 508, named after Section 508 of an American disabilities law. (Kinda odd naming considering Matias is a Canadian company.) You can't find the 508 on their website anymore, but they still have this one page site for it here:http://www.508keyboard.com/
It looks like the Half-QWERTY in a rubber dome format. I'm not sure if the 508 actually exists anymore. It seems that you end up ordering a Half-QWERTY if you click on the buy button. You can tell that the keyboard on the 508 page is different than the one ricer embedded. The proper 508 photo shows a keyboard body that's identical to their Matias Optimizer and various enhanced standard rubber dome keyboards.
Rumor has it that you used to be able to get a software emulator that would allow you to turn any keyboard into a Half-QWERTY board, but Matias sued to stop that product. I suppose you could just use a key remapper to achieve the same results though.