Ooh, thanks for sharing!
The middle, surprisingly, works well with Justin's vocals and Say It Right's background music. However, the ending seems weird. I wish there was a bit of Nelly Furtado actually in the song.
My all-time favorite mash-ups, that I have heard of and can remember:
Dandy Warhols vs. Beastie Boys - Intergalactic Friends
Beck vs. Pharrell - Frontin' On Debra