Just chiming in with my opinion, but having a powerful front light is far more important than rear. Getting hit from behind is pretty rare, especially in urban areas, although very deadly. Aspire to 1 strong headlight, a planet bike Superflash in the rear and Frogs for effect/backup.
BTW, these use watch type CR2032 batteries. These are raging expensive ($5) at Wal-drug, but I just bought 25 for $11 (shipping incl.) through a place on Amazon.com http://tinyurl.com/dczhzo
Finally, I think these are inexpensive, versatile lights that put you in compliance with the laws and it's cool to see hipsters getting lit.