I like the Blackburn Mars 3.0. Very attention getting strobe pattern -- amber sides to the bezel for some side visibility -- not too expensive ($15-$18) -- comes with a mounting bracket that would easily attach to a rear rack (even one w/o the built-in bracket).

I have also used the Cateye LD1000. A lot of people like these because they have two separate rows of lights, each of which is separately controllable so you can, for instance, set one to constant on, and the other the strobe. I thought it was good, but very expensive for a blinkie ($40) and about the size and weight of a beer keg.

Never tried the PB Superflash, and somehow have managed to survive years of commuting.