(The T-Rex site seems to be down for maintenance, so I couldn't link to it)
If you mean the stuff on this page, Chameleon Dust ICB from T-Rex's Sandfire Super-Foods range, I have used it, and there is nothing detrimentally wrong with it.
The only problem is that it doesn't quite fit into what most experienced keepers consider is a good supplementing regime.
That regime would be: regular dusting with pure calcium (no phosphorus, no vit. D3), less frequent dusting with a good multivitamin powder, and then occasional dusting with calcium that contains vit. D3 (the frequency of D3 dusting would be determined by how much unfiltered sunlight your cham is exposed to).
In conjunction with proper gutloading, that should be all you need to do to provide your cham with the nutrients it needs.
Now the Chameleon Dust has been designed to cater for all of these requirements: calcium, vitamin D3 and multivitamins; and it is intended to be used regularly - at all feedings. And that's the problem with it. Many of those vitamins (especially vit. D3 and vit. A) can build up in the chameleons system and have harmful effects. So you can't use this chameleon dust at every feeding. But you should be dusting with pure calcium at most feedings, so you'd have to get a separate calcium supplement for that. And if you're getting separate supplements, then the whole purpose of the Chameleon Dust is defeated.
So that's why I don't think it is really worth getting this stuff.
In my opinion, you'd be better of getting the Repcal or Miner-All products for calcium, and using Herptivite as a multivitamin.