d3o has been around for years, talk to any racer at your home mountain (well one that actually looks good) and they will know what it is. Spyder started using d3o in most of its stealths (what racers wear on their upper bodies to protect arms and back from gates) and race suits about 4 years ago. I myself back when I used to race always wore one of their d3o stealths pretty much every day I was training (you wear them under your racing suit).
d3o is awesome, its alot more light weight then the bulkier pads and lets you move more freely, The only draw back is its expensive. This means that you wont have a large amount of it to protect you if say you slam yourself on a rail. D3o is great at stopping impacts from fast moving and "softer" objects/material but if you are spending all day hitting rails a more traditional pair of padded shorts (like the ones red makes with big plastic/foam pads in them) are going to do a lot better job at protecting you. For a metaphor for instance, d3o is like a bullet proof vest, its going to stop a bullet, stop it fast, but your going to ge a bruise and some broken ribs. traditional padding is like a sand bag, it will stop the bullet slower and if you are standing behind one you will remain mostly unscathed. (D3o will still protect you well if you slam a rail with your leg regular padding just will do so better)
d3o = lighter weight, more breathable, great for falling on jumps. Great for people who are competent in the park
Regular padding = heavier, less breathable, can be bulky, great for slamming a rail. Also great for people who are going to be learning lots of new tricks and slamming hard all day long.
(dont take my word as if it were jesus's)