my Turnigy packs last around 200-250 cycles. I've owned about 30 packs, and have yet to recieve a bad one. They aren't 'b' grade in any way.
if you are going to pull high amps, go for the 30 or 40C, as they will run cooler and last longer. 20c zippys wear out after about 100 cycles when run near their max rating, especially in a hot australian summer.