One of the best duets I've seen and heard in a long time. I don't know much about Sara Bareilles (except apparently she's an a cappella singer who made it big), but she and Rivers sound great.
Weezer has really become one of the most creative bands out there — check out the AOL Sessions stuff. Chamillionaire, Kenny G (yes, it's true), covers of MGMT, Green Day, Coldplay and even Lady Gaga. Plus, Rivers' chorus on B.o.B's Magic is awesome.
Enjoy the video.