How good are penile prostheses for erectile dysfuncton?

Excellent & reliable. Penile prostheses are an excellent & reliable solution to erectile dysfunction if you can't tolerate medical options, whether oral, intraurethral, or intercavernosal, or mechanical options, like vacuum pumps. However, you also need to be a reasonable surgical candidate & willing to undertake that risk. Don't forget that ed precedes heart disease so be sure to look at overall health before surgery.
Effective & safe. Penile prosthesis surgery is reserved for men with ed that is refractory to all other non-surgical trestments. There is a small chance of infection, erosion, and damage to nearby structures, but most men perform effectively with a prosthesis.