Middle ear implants are surgically implanted devices. The FDA has approved specific middle ear implants and the FDA is still reviewing others. The middle ear implant is a useful hearing instrument and is quite different from traditional hearing aids.
In order to be fit with a MEI (or a binaural fitting), one requires a purely sensori-neural hearing loss. Since MEIs are better at generating mid- and high-frequency gain than low-frequency gain, the optimal hearing loss should be sloping. Many MEIs can be digitally programmed or are in fact digital. With the extra control that these technologies afford, other sensori-neural configurations can be fit. I would doubt if any new hearing aid users would be MEI candidates. Although the various surgeries are not complicated, they can be lengthy (up to 3 hours) and like any surgery, can be traumatic. A MEI candidate is therefore one who has tried conventional hearing aids and was unsuccessful either because (i) they were not able to obtain as much high-freuency amplification as required, or because (ii) the occlusion effect (Vagal response) could not be resolved to the satisfaction of the patient. While the cosmetic issue is important, this should be the primary deciding factor, given that CIC hearing aids can be made quite small with newer technology.