Ponazuril (bayer trade name Marquis) was originally developed for an equine parasitic disease (caused by a protozoan, Sarcocystis neurona) that affects the central nervous system (the disease is referred to as equine protozoal myeloencephalitis or EPM). Baytril is the "canadian" version although it is illegal to transport it to the US. Some people (mainly horse owners) have done so, but have been advised to do so at their own risk, and not do so in a car they would like to keep. As far as horses go there is really no advantage to using Baytril, but some people prefer to because it was what they used prior to the advent of Marquis.
In the horse world the cheapest that you can get it for is about $175 a tube (lasting a horse one week ... I don't know the breakdown but obviously it would last a cham much longer) and the price has gone up recently. I don't know if it has been looked at for safety in chameleons, but in horses is associated with very few side-effects and considered to be the safest drug for EPM. It is a cidal (so it flat out kills the parasites) although in horses requires a minimum of a 28 day treatment because it targets a specific stage in the life cycle of the protozoan which not all of them will be at at the same time.
For whatever it's worht ... obviously horses' are quite different than chams.
jamie