[cart213]

They are not that expensive next to a comparable top-of-the-line lipo, but they are expensive compared to a middle-line lipo such as Maxamps. Check out the stats below.

Compared to the Maxamps, the A123 has an advantage in capacity, current rating, and charging. Maxamps has an advantage in voltage, cost, and weight.

Compared to the Trakpower, the A123 has an advantage in cost, current rating, and charging. Trakpower has an advantage in voltage, capacity, and weight.

Maxamps 4200 2s2p (7.4V)
$77
201g
60A const. (15C)
80A burst (20C)
1C charging
$10.40/volt

Trakpower 4900 2s1p (7.4V)
$125
268g
98A cont. (20C)
147A sustained (30C)
245A burst (50C)
1C charging
$16.89/volt

A123 4600 2s2p (6.6V)
$100
310g
138A cont. (30C)
276A burst (60C)
2C charging
$15.15/volt

The biggest disadvantage of A123 (imo) is the lower voltage rating. However, this can be overcome by increasing the pinion size, as demonstrated by Mike and Batfish.

The biggest advantage of the A123 (imo) is the safety and durability. To be honest, I am lazy with the care of my batteries. I leave them in my car, my garage, and my desk at work, and I don't like to monitor them during charging. I wouldn't feel comfortable treating lipos this way, so I'd rather pay a few dollars more for the extra peace of mind.