The breach was initially thought to be massive; The Register reported that the leak consisted of around 32TB of files. They claimed it included builds of Windows that haven't yet been released. However, it later became clear that the leak was smaller than originally reported, and what's more, much of this data had been made available. The Shared Source Kit has already been distributed to Microsoft's partners and licensees through the Shared Source initiative.
That doesn't mean this data leak isn't serious, though. It's an embarrassing black mark for Microsoft at a time that more and more people are paying attention to and concerned about computer security. While the source code has been removed voluntarily by Beta Archive, it's unclear how many people had already downloaded it. It's possible that it still could be distributed via other methods and used to create exploits for Windows 10.