Huge feature from Facebook. For the end-user it looks great and it makes perfect sense: clicking on a link in the newsfeed is not a good way of consuming content. This is.
Facebook has a lot of ambitions with this instant articles feature, which takes a lot from the design of Facebook Paper. If it takes off (and I'm sure it will), this could one day become a serious threat to Twitter.
There is also the question of how do publishers make money. Here they do not use their websites to display ads, they are counting on Facebook. It can work, as long as everybody is happy. I think this is definitely a good way to monetize content on mobile, where Facebook dominates ad revenue and where publishers struggle the most.