Right now, I'm using that keyboard's less wealthy sibling, the kinesis freestyle. I've found that it's a lot more flexible, as I can stop, do other things, like use a drawing tablet or mouse, without the needing to put my hands in the bowls like i would with the advantage. Also, I have smaller than average hands, so I get the feeling that the advantage would not be as useful for me.
Now, what I'd really like to see is keyboards made with different key sizes. A 3/4 size keyboard that has proper mechanical switches would be very, very nice indeed. Hell, I actually like 15" laptops with full keyboards because that tends to mean that they've shrunk each key to fit, and it gets to right about the sweet spot in size for me.
I love the freestyle. First keyboard I've used without hand pain. I recommend rolling a t-shirt or putting a foam pad below your wrists to prevent strain.
I got the accessory kit with the wrist rests and slanting mechanism at the same time as the keyboard, so that part's not an issue. I got mine before I've started having wrist pain as a preventative measure. I'm 35 with weird genetic conditions regarding my joints, I don't need to exacerbate things.
I think the other piece of the puzzle is that a lot of people don't properly adjust their chair and/or monitor for good ergonomics either. Eye level with a monitor should be right about at the top of the screen. It takes a tiny bit of getting used to at first, but then you really start noticing how much less neck strain you have.
Now, what I'd really like to see is keyboards made with different key sizes. A 3/4 size keyboard that has proper mechanical switches would be very, very nice indeed. Hell, I actually like 15" laptops with full keyboards because that tends to mean that they've shrunk each key to fit, and it gets to right about the sweet spot in size for me.