Methadone has some serious issues with it that patients need to understand before starting it.

1) it prolongs the QT segment of the heartbeat and in people taking other drugs with this action, it can be additive and stop the heart. Some people have a genetic inheritance to have long QT anyway, and would react to methadone with slowing of the heart beat. So getting an EKG reading before starting is a good idea to rule this out. And also keeping track of other meds with this potential and not mixing them. (ex. amitriptyline.).

2) if you get low in potassium and magnesium, then this can tip a normal person taking methadone into a QT event:

here is a link about it:
http://crediblemeds.org/

3) Methadone stays in the body for a long time. It leaves the pain receptors BEFORE it is completely gone or ready for next dose. So if you take it too soon or too much you can become overloaded on it, and get either overdosed, or a QT event.
http://en.wikipedia.org/wiki/Methadone
half life is 36 to 55 hrs... meaning 1/2 is gone after that time.
So when you start up you have to be on it for a while to get a rhythm going to cover your pain.
Overdose of methadone could suppress the breathing reflex.
The pain receptors will be empty but the methadone is still in your body affecting other organs. This is why it is considered
serious and most doctors are not equipped for understanding these technical aspects. (pain management doctors should be better in this regard.)

As far as pain relief, it can be very good. It has less euphoric effects, and is much less expensive than other opiates. But people thinking of using it should really study it well, beforehand, to avoid accidents.