I haven't used them either, although I have checked them out on e-bay. I do like the concept, but also prefer a cage that I know is secure. I have gotten my last cages from DIY, and am very happy with them. They have three sizes and can be put together in very little time. They are also cheaper than the instacage and offer free shipping. Sounds like the instacage wouldn't be bad if they get the bugs worked out.
This is a great idea especially for shows and quick homes for small chameleons or if the cage is being clean. Something in a rectangle with magnets should be incredibly strong actually, but only once it is completely together.