A123s can handle high charge and discharge rates better than any lipo out there (atleast on paper so far). After 1000 20C discharge cycles they are rated to only loose about 20% capacity. With heavy abuse of 30C discharges and 10C charges, they're still rated to last about 200 cycles! If charged at 4C they're rated to last 1000 cycles or more. Charged at 0.5C, 5000 cycles! No Lipo can claim anything even close.

Add to that they're very durable under physical abuse and tolerant of charging and discharging abuse. In a pinch they can be charged on a standard lipo charger as they will tolerate being charged up to 4.2V per cell.

Weight is their main downside IMHO.