TrueRc's 8000Ah 4s pack is very good for the price ($140). I have 2, and have put them through hell and they have never failed me. I even rounded the end of the cells a little and they are fine. I hit a pot hole I didn't see with such force, that it bent the end of the cells against the battery box and shifted the cells in the pack. The lipos are ok though.
As far as bang for the buck, I don't think anything beats the A123 cells. DeWalt packs can be had for $110 and will net you 10 cells. A little time on your part and materials and you can have two 5s packs (that will perform simialarly under load as 4s lipo) for $125.
I have some of the 8AH TrueRcs as well. They are really nice, but they are just a bit too wide @50mm fit everywhere. Other than that they are great.
I like the idea of hte A123s too, just they are a bit too big and heavy and low runtime for me to take the plunge with them now. The next gen ones should be really awesome tho. They have great potential.