I've seen two or three of them. I was just reading about them earlier this year, and they claim that they are more efficient. The big savings comes in if you live in a cold climate. The dryer is not exhausting the air from inside your house to the outside at a rapid rate, causing your heating system to work more than it would. They actually cost more to operate than a normal dryer, but when you factor in the heat loss from a conventional set up, it was a lot closer than you think. And, if you can't vent to the outside, you don't have a lot of choice anyhow. I personally don't see myself replacing mine, since it doesn't actually pull heated air from inside my house, but it's a nice option in some situations.

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Jim Robinson
New Mexico, USA