The ADA glass diffusers are more durable in hand then they appear, and stronger then the knock off brands. But as Roy mentioned, it's hard to justify spending excess money on something that a much cheaper product can do at a reasonable effectiveness. In my experience with both types, the bubbles are only finer on the ADA ones during the initial use. After a few days, it ends up dirty and produces about the same as the other brands.
I'm not sure if they are nicer looking then the alternative brands either. In my opinion they look the same, and most people who visit my home don't even notice it. So if the function is the same, I think in the end it all comes down to preference. Like $400 Coach Purses compared to Chinatown's Coach Purses.
-John N.