We don't really do drug comparisons here.. I suppose I'll let this stay open for a little while in case I miss anything but..
Basically, there are opiates and opioids. Opiates (morphine, codeine, Thebaine) are all alkaloids derived from Papaver somniferum (the opium poppy), from these alkaloids, semi-synthetics are created such as oxycodone (codeine and Thebaine). These drugs fit into the same receptor sites which endogenous opiates (endorphins)
Methadone is an opioid, meaning it is completely synthetic, but still acts upon these same receptors. Methadone is a very strong opioid compared to others and it also has an especially long half life. For these reasons, some people enjoy methadone for recreational purposes, as it is potent and will keep you high for a long time. This also makes it very dangerous though, and when mixed with other CNS depressants such as benzodiazpine's and alchohol (which unfortunately is common practice) It is rather easy to overdose on. Another thing about methadone, is that with chronic, high dosing (60mg and above), methadone raises ones tolerance to a point where opiate receptors become saturated, creating a blockade effect (which is one of the reasons that it is used for opiate replacement therapy.
At a stable, high dose of methadone it becomes nearly impossible to get high on opiates or opioids, even on methadone itself. Many people will try and get a buzz back by doubling there dosage, but instead of feeling euphoric, they become sedated and run a serious risk for resperitory depression.
People often discourage recreational use of methadone for these reasons. I also noticed that you tagged your thread with a syringe. I'm hoping that you are not injecting either of these substances, especially opium which is full of all kinds of plant material. Injecting methadone is also pretty pointless as it has a very high oral BA, and IV provides no rush. Not to mention that without a filter, your introducing all sorts of harmful pill binders directly into your blood stream.