I run them basically because I was able to source the cells out of 36v Dewalt power tool pacs and the pacs I built were incredibly cheap. My 5S1P pacs cost me $55 each with the shrink and balance wiring. They are heavy - not quite as heavy as a 6 cell NiMH pac, but close. They are also big - there will be places you can't run them or have to configure them in tubes to minimize size - as a side by side pac, they're larger than 6 cell NiMH in sbs.
They are lower voltage, 3.6v as opposed to lipo at 4.2v/c. Generally not a prob - if you would run 4S lipo, you run 5S a123 since its similar voltage. To generate capacity, I built pairs of pacs - when I run 5S1P, I use 2 wired in parallel so I have 4600mah capacity. I charge my a123s at 10amps and they're usually ready to run in about 20min, about 1/3rd time of a lipo. Also the graphs I've seen on life vs charge voltage tell me to expect about 1000cycles, so they're supposed to last longer than a lipo. Right now, if I had to buy cells heads up from A123, not sure I'd go that route.