Methadone isn't really a research chemical but its one of the few opioids I have no experience with.
MDPV on the other hand is a beta-ketone based on proveralone, with a 3, 4-methylenedioxy ring added and is said to be safer than its parent compound. It's highly addictive, 10-20 times as potent a dopamine/norepinephrine reuptake inhibitor than cocaine, and to someone with no tolerance is active in doses as low as 2.5mg.
It can be ingested via various routes, my suggested dosing regimen for the following:
Smoked: 2mg
Oral: 2.5-5mg
Insufflated/Intrabumbum: 2-3mg
IV/IM: 2mg
Smoking is not advised, as it can produce a hazardous carcinogen called pyrrolidine, it is questionable as to whether or not this chemical is present in 100% pure samples, though it's hard to know if it is pure without proper GC/MS testing. This chemical is probably present regardless of route, but smoking is also one of the most addictive ROAs, followed by IV and IM (depending on person).
The onset/duration for those routes (in order) are approximately 15s/30-60min, 10-30min/2-7hr, 1-5min/3-4hr, instant/3-4hr.
Most users of this compound report that its effects are highly highly variant. As in, your mileage may vary.
When I use it, I find extreme compulsion to redose (through vaporization), though I rarely do. Orally is my preferred method.
The effects I report along with a 1/10 rating are:
Brief to moderate euphoria rushes (5/10)
Increase in concentration (10/10)
Peripheral stimulation (8/10)
CNS stimulation (8/10)
Those are the positive effects.
Sometimes the euphoria is great, sometimes, it's not even there.
I hardly consider it recreational, though it would make a great study aid for people with ADD.
Negatively, its a vasoconstrictor, increases blood pressure, causes palpitations, and a host of other cardiotoxic symptoms that increase with dose.
Only someone with a LOT of self control should ever use this, and I don't think anyone should ever use this to be honest but if you must that's my advice
I quit a long time ago, because if you redose the duration becomes additive but the positive ones stop showing after the first dose generally.