I would like to play around with them very much, but they are awfully expensive.
I ordered a few LiFePO4 cells from DX; I was thinking that since the technology is technically the same they'd have the same performance. Wishful thinking; 1300mAh LiFePO4 18650s can't deliver more than four amps without overheating. The C rating is higher than standard LiIon, but since the capacity is so low any 2200-2500mAh LiIon 18650 can knock them into a cocked hat.
I've searched eBay for genuine A123 cells, but they want something like €15 for EACH cell. I think not!