Today the FDA approved the first completely implantable hearing aid, which is designed to treat sensorineural hearing loss.
According to the press release and this LA Times blog post, the Esteem is a hearing system - with a sound processor, sensor and driver which work in concert to assist the patient with their specific hearing loss. The device has no external components. There are some possible side effects from the surgery necessary to implant the device, which can include facial paralysis and taste disturbance. (I'm not an expert on cochlear implants, but from what I've read they can also cause the same problems due to the surgery.)
The Esteem is for adults 18 and over and can be used to treat stable sensorineural hearing loss - I guess progressive loss wouldn't make you a good candidate for the surgery. Your inner ear has to have normal anatomy and, according to the article, "[a] patient's ability to understand speech using Esteem should be similar to that of conventional hearing aids."
So it sounds like the Esteem is an alternative to hearing aids. I can see the desire for a hearing aid that is completely hidden, but I can't see the same subset of people who want an "invisible" aid being willing to go under the knife for surgery to get it, when normal hearing aids would assist them just as much. The FDA article says that 93% of the people who got the implant in the clinical trial "scored equal to or better than their pre-implant hearing aids on a speech intelligibility test." I'd be interested to know how many of them were equal and how many were better, because I don't know if I'd get surgery just to have hearing the same as it always was with aids.
This really is pretty cool. To be able to have a hearing device that is unnoticeable from the outside is a neat achievement for science. There are many ways to approach the subject. For example, some people who wear aids like to make them prominent so that people will know they have a loss and adjust what they do accordingly. The Esteem would mean no one would have an idea you had a loss unless you told them.
It's definitely an interesting advancement. I'm looking forward to learning more about it as more people get the implant.