It's no placebo but if you're dependent on a full agonist you will barely feel it. Recently I found myself addicted to poppy pods and kratom doesn't even touch it. barely helps prevent withdrawals. But if you're not addicted to a full agonist, it gets you high and is quite pleasant, and produces withdrawals that suck. Using kratom here and there, it can be quite pleasant, in fact it was how I got into opiates in the first place, I used to LOVE kratom, back when I was an opiate virgin. I used to get an awesome high from it when I dosed a half ounce at a time using the traditional tea method (discard the leaf material, this allows you to dose higher without experiencing the dizzy/nauseating side effects). But dose it every day and it starts to suck and you get withdrawals.
Anyway there is no debate on whether kratom is an opioid, mitragynine and to a greater extent, 7-HO-mitragynine, are partial mu opioid receptor agonists.