Please do not mix methadone and fentanyl, that's like playing Russian Roulette with THREE bullets in the chamber.
If you've tried all the other opiates, trust me you're not missing out with fentanyl or methadone. They are awesome medications for pain and maintenance, but in terms of recreational value it is low because the dose/response curve is so steep that there is a VERY fine line between recreational dosing and enough to OD. Both provide much more sedation and respiratory depression than they do euphoria.
If you absolutely must abuse fentanyl, do not mix it with methadone, and start low (I would suggest no more than 12.5micrograms for the first time. The biggest problem is that you have no way of weighing such a small amount, without laboratory grade equipment Then you would slowly titrate your dose to where you want to be in increments of 12.5 micrograms. And then try to remember to breathe, I on fent I would catch myself holding my breath for longer than thirty seconds and not noticing it, with overwhelming sedation, and minimal cold euphoria. Everything about it feels synthetic, don't do it.
Honestly, both have zero recreational value. Methadone, at least is a potent analgesic and has a very solid role in the treatment of pain management, and it's potency, duration, half life, NMDA antagonist action, and no ceiling dose, and is highly effective for when a chronic pain patient needs to rotate medications, which usually will happen more than once in a CPPers life.
Methadone is also an incredible drug that is used for Methadone Maintenance Therapy, (usually after Buprenorphine Maintenance Therapy has failed or was not justified for another medical reason) and allows thousands of addicts to live a safer HR lifestyle (excluding those who abuse the system of course, of whom there are great numbers) by avoiding withdrawals so the addict in recovery can slowly transition to a healthier lifestyle, hopefully recovering and refraining from IV drug use.
The system is not perfect, by any means, as we all know, but they aim to re-integrate you back into society through mandatory psychotherapy (depending on the clinic) so that you can focus on getting a job, or go to school. Buprenorphine Maintenance Therapy works for some too, but everybody's different, which is why we're lucky to have methadone and buprenorphine. It's nothing like how certain progressive countries allow for heroin maintenance, morphine maintenance, doing trials with hydromorphone maintenance therapy, etc. But at least we have more than just methadone, which really should be the last resort. Methadone is one hardcore opiate, it is so silly to hear about people with a hydrocodone or oxycodone habit hopping straight on methadone...
IF IT WERE UP TO ME: I would say every addict would be treated with their DOC ( slowly titrating down with their dose administered in a supervised setting with fresh equipment). When I was getting off an insane high dose alprazolam addiction, the protocol diazepam, clonazepam, IV lorazepam etc, none if it worked except for a slow (over a year long) taper with alprazolam.
^This should not cost more than the daily dosing fee at a methadone clinic, all your equipment cost less than your daily dose.