Brilliant for the guys who founded Mojang, bedroom programmers to billionaires in six years, living the dream.
Dumb as rocks for Microsoft, this is just like EA buying developers at the heights of their powers and then wondering why explosive growth doesn't appear by magic. Minecraft is at the top (or arguably slightly past) of it's sales curve and Mojang has no other massively saleable products (Scrolls is no Minecraft), so how do Microsoft plan to get their billions back?
Making Minecraft 2 as an Xbone exclusive won't do it, nor will shovelling micro transactions into the next two years of mandatory Minecraft updates. The three guys who made Minecraft are leaving too, so there goes their much desired lightning in a bottle.
Microsoft will make no money from Mojang, they will try to leverage cash out of it as quickly as possible to get a return on their investment (use your imagination as to how), the number of people playing Minecraft will tank, Microsoft will have Mojang do nothing but Minecraft, then they will be quietly closed down sometime between now and 2020.
That's what happens when large corporate entities invest massively in the last big thing instead of looking for the next big thing.