1st off,
methadone does not block the effect of opiates, like naltrexone and other opiate antagonists. It is mearly a very long acting opiate agonist, like
morphine or heroin but 2-3 times longer in duration. this is why it is prefered for heroin treatment, because it only requires one shot a day to avoid wd rather tham 3-4 for heroin. This also means that it has a much longer wd period than heroin or morphine however, and people coming of of methadone have described WD's lasting a month or more. So mixing methadone w/ the oxy in the percs won't send you into instant OD like
suboxone would, but I would be careful using them for breakthru pain b/c methadone stays attached to your opioid receptors longer than you may think making OD more likely.
as for your second query, many heroin users say that injected methadone provides less of a rush/euphoria than heroin or morphine, but this may in part be a placebo effect since the set and circumstances of getting high in private and getting high with a doctor in his office as part of a court ordered drug rehab can heavily alter the experience. blind trials have shown that patients cannot tell the difference between morphine and methadone on the initial hit, but the harsh WD's of methadone keep it from being a favorite among abusers of narcotics.
Information = Freedom