I use A123/LiFe for my rx packs with no regulator and they work great. I'm still amazed at how long the 1100mah 2 cell pack lasts. The discharge curve is really flat. It drops to roughly 6V quickly and then it basically stays at 6V till the pack is drained.
I have a friend that tried to use the larger A123 packs for 1/8th off-road and they didn't hold up well. They worked great for about 3 minutes and then they were done. Not sure if it was just the packs he had or what but he switched to LiPo and is running great now.