It's an experimental feature. Downloads will be generated at an extremely slow rate to avoid being blocked by Microsoft.
From the tests I've made, I'm confident that it's going to work, but only at a rate of one download per hour, for about 20,000 daily users... Therefore, chances for people to get the exact download they need might be a bit low.
But if you need an Ultimate download, you can still go for a different edition (Home, Pro), and convert the iso file to an all-in-one file.