I offer one meal of mealworms every week or so.
They are not a terrible food, but as a sole food source, definitely not the best either. They gutload calcium better than crickets (see ferguson panther chameleon book).
They have been used as the sole food source of commercial leopard gecko operations over generations with very long lived geckos. (not a good idea for chameleons, but demonstrates they are not nutritionally worthless).
Ferguson used them in combination with crickets to provide a nutritionally complete diet for his panther chameleons in a laboratory setting over multiple generations. He chose them for the calcium content when gutloaded properly.
Like any feeder their nutritional value depends on how they are fed themselves.
People tend to look at the exoskeleton and worry about intestinal blockage- I don't think that could ever happen unless you have a sickly lizard. Chameleons thrive on invertebrate prey with exoskeletons- ability to deal with exoskeleton is just what they do.
In my opinion, they are a good part of a varied diet.
But only one part- variety is key.