However, it's diverged to have little in common with either AOSP or Cyanogenmod in the past few years. They more or less try mimic an iphone-like experience on Android, only with more customization and features.
I'm not a fan of it really due to the fact it's closed source and I won't touch any Android ROM that doesn't post its source. Quite a number of people like it, but there's many other options out there.
However the ironic part is the areas of the Android source they must disclose under GPL, they avoid sharing. They don't release the kernel for any of their devices as well and that's a huge GPL violation. I would presume if they did release it for their own phones, it would be on here with the rest of their stuff. The rest of their official sites (there's a lot of non-affiliated English and other language sites that label themselves as Miui) are in Mandarin and much harder to navigate if you're an English speaker.
Doing a google search for Xiaomi MI2 kernel source, turns up nothing positive about their kernel or about them and GPL though.