It FIXES a problem with IE7 failing to download some files when using a Japanese language setting. It is noncritical, can be worked around, and will presumably have a corrected version out shortly.
It has CAUSED a BSOD error in half a dozen machines, to my personal knowledge alone, this morning, most with NVidia motherboards.
And if I know about 6 of them, that means there's probably 600,000 of them.
So save yourself the headache, and don't install this update today.