Mystery Capsules give you 9 exclusive items, and sometimes more. You can buy them at the NC Mall.
On June 11, 2014, all of the existing Mystery Capsules were retired, and in their place, new capsules were released that focused more on exclusive items and older re-releases.
The new limited edition item capsules no longer award a random NC item as the old style capsules did. (But opening an old style capsule will still function as it did!)
Old style capsules might also give you an exclusive capsule bonus item which can't be bought or a rare item code which can be used to unlock the Space Faerie challenger.