Having known somebody on methadone... it's not great. It's just as addictive as heroin, but instead of making you feel good, it makes you feel like shit. And then if you don't take the drug that makes you feel like shit, you go into withdrawal again.

If the goal is to punish somebody for the rest of their life for having fun on an illegal substance, then mission accomplished. If the goal is to remediate the user so that they can be a productive member of society, then it's an utter failure. Sometimes people don't get hooked on the methadone itself, but it's a very delicate balancing act and not at all guaranteed