Syndrome pretty much nailed it word for word.
Top guys at our track are running nimh's, lipos are close, but seem to be used by people who dont want to hassle with the maintenance.
I run a lipo, an Orion Plat 4800. The only thing that worries me is I dont want to be the first guy that has a problem with one on the track. Mine has been pretty bullet proof, had its shares of crashes and has come out fine time after time. I just wished they made it heavier, it being lighter isnt an advantage when I have to add a ton of weight to make it legal and balanced.