One of the worst experiences/trips of my life occurred from taking Suboxone after being on methadone for months. I had waited 2 days for the methadone to get out of my system before taking the Suboxone, but apparently that wasn't long enough.
I was thrown into the worst withdrawals I had ever experienced

It was extremely unpleasant to say the least.
Hopefully methadone is in the process of being phased out in favor of Suboxone/Subutex. I've known people who have been strung out on methadone for 20 years. In a lot of cases methadone clinics resemble government sanctioned dope houses IMO.