While l-methamphetamine is psychoactive it isn't reinforcing, or at least not particularly reinforcing.
It is mostly a norepinephrine releasing agent, at around 15x the strength of its dopamine release. Thus, a DA/NE release ratio of around 15, which is similar to ephedrine -- which is perhaps slightly more specific of a NE releasing agent. By comparison, d-amphetamine has a DA/NE release ratio of around 3.5, and d-methamphetamine is around 2. Optimal for a recreational drug (at least in my estimation) is around 2 to 1.2 (independent of serotonergic activity). Because of this l-methamphetamine actually reduces the reinforcing effects of d-methamphetamine, so dl-methamphetamine is actually somewhat less reinforcing (although it is still very reinforcing even as a racemate).
So l-methamphetamine has pretty similar pharmacodynamics as ephedrine (which isn't surprising since they have virtually the same structure). I have never taken pure l-methamphetamine (though i probably should just out of curiosity), but I have taken alot of ephedrine, which is a fairly pleasant functional stimulant. On paper, l-methamphetamine appears as though it would perhaps be a bit more enjoyable than ephedrine.
As far as racemic meth as a substitute for d-amphetamine in the treatment of ADHD, I'd say it is inferior in most cases due the side-effect profile and abuse potential. I have tried racemic methamphetamine as a substitute for d-amphetamine for this purpose and found the quality of focus to be worse.
Yet even pure d-methamphetamine, which I've had in the form of desoxyn, felt inferior to d-amphetamine as an ADHD drug. I found desoxyn to have a dreamy, spacey quality to it that made me feel lethargic, cold and tired due to it's comparatively weak NE release and increased serotonergic activity. I tried it for a month and then said fuck it, particularly since my insurance refused to cover it (essentially saying, "nope, no way we're picking up the tab for your sack of meth bro") so I had to pay $400 out of pocket. Had they covered it i would have probably tried it for another month just for the sake of curiosity and given it's rarity.
In that sense I actually feel racemic meth might make a better ADHD med since the increased NE release helps offset the dreamy, spaced out effect of the serotonin release of the d-isomer. Overall, serotonin release makes ADHD worse, which was abundantly clear when I sampled 4-FEA, an amphetamine with highly specific serotonin releasing properties, which makes one feel very cold, tired and spaced out.
But ultimately, outside of this theoretical discussion, the real story here is that using meth, especially street meth, as an ADHD medication is in almost all cases a bad idea as it has a tendency to go south very quickly. Even pharmaceutical methamphetamine is generally worse due to the anti-therapeutic effects of serotonin release and associated increased neurotoxicity (when compared to d-amphetamine). Pharmaceutical d-methamphetamine may be better in unusual cases, like in patients with austism spectrum disorders or with behavioral issue, like to chill out the screaming kid that lays down on the floor at the supermarket and throws a tantrum. "Now come on Grayson, take your desoxyn"
