Another study on methadone’s cognitive effects:
In one study, on the day after the last exposure to methadone, there was a significant reduction (around 70 per cent) in the level of a signal molecule which is important in learning and memory, in both the hippocampus and in the frontal area of the brain. This reduction supports findings from a previous study (Andersen et al., 2011) where impaired attention in rats was found at the same time. At this time, methadone is no longer present in the brain. This indicates that methadone can lead to cellular changes that affect cognitive functioning after the drug has left the body, which may be cause for concern.
The study was done on rats, not people, but it’s findings aren’t isolated.
Patients and their loved ones may decide that the potential benefits outweigh the costs, but they should be made aware of the costs and know the alternatives.
Addiction & Recovery News 
It’s important to note that none of the three studies linked here show causation – at best, correlation. The two human studies measure against healthy controls, meaning that the outcomes could just as likely be moderated by a prior history of drug use, as by methadone or suboxone. Likewise, in the rat study (a clinical subject with tenuous implications for humans) it isn’t clear that methadone is unique among opioids in causing this type of change. It is well known that opioids cause long-term changes to the endogenous opioid system: http://m.physrev.physiology.org/content/81/1/299.full
Opioid addicted health professionals tend to use large doses and go undetected for a long time, and they have great recovery rates.
There’s no reason to believe other opioid addicts would experience impairment that health professionals do not.
There have been studies looking at methadone maintained patients vs. abstinent opioid addicts. See here and here.
Same thing with buprenorphine.