I don't use it (currently).
I faintly remember
looking at it. If I'm not mistaken it means additional markup. I'm not sure how many user agents implement it yet, or more importantly how much. If some did it would, I guess be required (at some time) by goverment type work, and specialised applications. It looked like a lot of work following a cursory evaluation.NOTE:
I decided to get a bit more insight and found this link http://j.mp/CqISE
. It mentions some special UA's and notes that Opera 9.5, Firefox, Safari and IE8 have some sort of implementation. (The implementation might be very small, I haven't tried to check.) If a developer has to pay for a UA that's not going to speed adoption.