I've seen a couple of case studies about single users using kratom to get off of stronger opioids, but no at-scale studies. But there is a reasonable amount of other literature relating to kratom and its primary alkaloids, mitragynine and 7-hydroxymitragynine.

Of particular interest is that these alkaloids are G-protein-biased agonists of the mu-opioid receptor - which also do not recruit β-arrestin[1] following receptor activation. It is β-arrestin recruitment that is responsible for the main side effects of opiates - respiratory depression, constipation, histamine release and build up of tolerance (which is what leads to withdrawals upon cessation). This is a very big deal.

I am a kratom user myself, using it to treat bad neuropathic pain. I am prescribed opiates, but kratom doesn't space me out like they do, there is no respiratory depression, no apparent histamine release, the constipation is much less, and I've been at the same dose for over a year. Honestly, for me it's a wonderdrug.

[1] http://pubs.acs.org/doi/abs/10.1021/jacs.6b00360?journalCode...