I think a combination of placebo, satisfying your needle craving, and the burning of your veins is the "rush" you experienced. One of the reason they use methadone in opiate treatment programs is because it gives no rush due to it having a high volume of distribution, which means it dissolves into all your tissue before it gets to your brain.
Hardly a placebo heheh. Have you ever shot a ml or two of the 50mg/ml ampules? Crazy rush. Not like heroin, but certainly getter than oxycodone. Spend a second on google....my experience is far from unique. It does vary from person, and like I said many people try shooting the far less concentrated liquid or tablets and don't achieve much.
High volume of distribution, sure, especially orally, but when it slams into your brain seconds later obviously it is "not all dissolved into your tissues before making it into your brain". Quite the contrary!
I took part ( as a participant) in a study comparing the efficacy of IV methadone vs intranasal vs oral vs nasal many years ago whilst an undergrad at the UW....they did a lot of studies on the different ROAS of methadone, as well as the different racemates....good stuff. I'll try and dig some up later...but anyways, as the participants were supposed to be non opioid tolerant (ehhh....) they had to do a one minute push on the injection, as the roughly 10 mg dose (dependent on body weight) was enough to send most people barfing...they eventually slowed it to a 3 minute infusion and the rush of such was still too much for most. Not I!
Below is a PDF of a study on IV vs oral methadone....more about lifestyle differences, but some interesting stuff on the pharmacokinetics of such...a considerable amount of methadone is lost to first pass hepatic metabolism, and the oral bioavailability varies more than most think....
http://www.ahrn.net/library_upload/uploadfile/sw0001.pdf
Conclusion? The right preparation and concentration of methadone has a fantastic rush for most people, especially if you are not already saturated with the stuff...