[MetalMan]

I switched to Lipo almost 2 1/2 years ago, and have never regretted doing so. I'm always on the prowl looking for the best deals on Lipos to maximize the bang-for-your-buck factor. For my Revo, I chose A123 packs because they are so cheap (in Dewalt packs), and because they still outperform NiMh by quite a lot. Here are advantages of A123 over NiMh as I see them:
-MANY more cycles
-greater power
-less weight (5s2p A123 weighs ~720g, 12 NiMh weighs ~800g)
-higher voltage (5s=16.5v)
-higher capacity (2p=4600mah)
-VERY consistent power until the last second
-FAST charge times (with the right charger)
-safer (won't "explode" at all like NiMh can)

But for competition, Lipo definitely has A123 beat for power and energy density. That's why my Hyper 8 is running a Maxamps 4s 5000HV Lipo pack.