Yup, Larry Niven's "The Ringworld Engineers". The main protagonist uses a "droud" (as the implant is called in the book) to directly stimulate the brain with electrical impulses.
He only disconects the droud because he knows he will die from starvation, dehydration and atrophy if the doesn't go through the motions daily. But every moment without the droud is existential dread.
I don't know about other users wasting away, but I remember Chmeee (of the tiger-like species) destroying the droud when they were travelling to the Ringworld, so Louis Wu was without an option on quitting it.
