Hmmm, I don't recommend them, but many people use them here and have lots of good things to say about them.

I personally don't like them because of the aggression that can come from some birds who use them. Not all birds act this way, they work for some but not all. For some birds, they see it as nesting and will become aggressive when you want to take them out in the morning. I know someone who used one for their Grey; put it in a dark place where they could get their uninterrupted sleep, but became crazy possessive of it and wouldn't even let her owner touch her, it turned out she thought it was her nest.
We don't use them and Casey does just fine using her cage. They aren't a necessity, if you really
want to use one you could try it out and see if it works for your bird, if aggression comes from it, go back to using the cage.

Bumping it up for other personal experiences.