Dr. Drew Pinsky, said that the vast majority of methadone maintenance programs are conducted improperly. He even pointed out some that were gradually INCREASING the amount given, when the idea is to ween a person off of it. If done right and managed right, it's irreplaceable, no system works better, but a person HAS to be monitored almost constantly to make adjustments. Unfortunately, there just aren't the resources to do so. Add to that some use it just to hold them over while they're out of H, with no real intention to quit. These people often end up addicted to both at the same time, making matters worse, in that methadone's relatively easy recovery is absolutely stonewalled when coupled with heroin use.
For many addictions, replacement therapy is WAY more successful than cold turkey. Some drugs, and alcohol and heroin two of them, can kill you if you just stop taking it. Darksider is right, it's trading one vice for another. BUT, often the vice you're trading to in replacement therapy a) keeps you from dying b) get's you over the withdrawals, or at least makes them manageable c) is an easier vice to quit.
The same principal us seen in using nicotine gum to quit smoking, which was absolutely paramount to my own quitting of that habit. I quit smoking after 20 years of it, no stumbling or cheating (PM Me if you want to hear the whole process).
When you realize how addiction works, specifically opioid addiction, you see how hard it is to get over. Your body produces opiates and has opioid receptors. When an opioid receptor detects a opiate, you feel good. A normal body should have some degree of that working at all times. Less when you feel sad, more when you're happy or excited. Do an opiate like H, and you feel ALOT of it. Do alot of opiates often for long times, and your body stops producing opioids, and allows opioid receptors to die, so without the drug, your body has less of it, and less of an ability to detect what is there. Then, when your body is on average, you are feeling the same as someone who is really sad or depressed. That is also why the first time you do heroin, it's REALLY powerful, and you never quite get that high back, because even with light use, your body starts to slow the normal production/reception process. It gets to a point where one has so few opioid receptors that they have to do a sizable hit of H just to fell decent. Not GOOD, not GREAT, , just DECENT. That is a tough hurdle to get over, because a person recovering isn't feeling AVERAGE, they're feeling TERRIBLE due to seeing where their life is and everything that is wrong, how they let people down. Their body is making LESS opioids than a depressed person's is, and has less receptors to detect them. That is why recovering addicts speak of feeling hollow or dead inside. Your level of opioid response is supposed to go from low reception for you being sad, to high reception when you're happy or excited. The recovering addict feels NONE, or almost none.
Unfortunately, one of the big parts of recovery is to get out of the environment that the addict was in when they were doing that crap, which isn't an option for many.