In the wild, i believe a panther chameleon would encounter more praying mantis in his life than crickets.
Yes, it's a good feeder.
But, i can see why mantises quickly become nominated as enrichment feeder than staple. It is the inefficient way of gutloading them.
First, you got to get a good gutload. Second, feed the gutload to crickets/ roaches/ worms/ etc. Then, feed the gutloaded insects to your mantis. Finally, feed your mantis to your chameleon.
I do agree that mantises are 2nd most interesting creatures (with chameleon holding #1). So, you can get attached to them...
Mantis place is a good site to buy a mantis pet.
But, you would soon get bankrupt if you buy one mantis to feed it to your cham.
Buy the ootheca that can yield about 200 nymphs. Ebay sell them cheap, local nursery usually carry them, and Online nursery site also have them (usually under beneficial insect tab).
You can feed them all to your cham. Or, release some in your backyard (Provided you bought the chinese mantis or european mantis religiosa), keep the rest and raise it a bit bigger then feed them to your cham.
Just know that the bigger they are, the tougher they become. Mantis is a fighter insect. (But, you should not under estimate the ability of your chameleon as well).. all those years of evolution, your chameleon definitely know an efficient way to kill a mantis without getting hurt. I have on rare occasion feed mine with an adult mantis. And, honestly, the mantis did not even stand a chance.