http://bjp.rcpsych.org/content/187/6/510.short
http://www.tandfonline.com/doi/abs/10.1080/02791072.2007.10399879#.VSJguvnF9fc
http://online.liebertpub.com/doi/abs/10.1089/cap.2008.033
Conclusions: Bipolar adolescents with CUD demonstrate evidence of greater structural abnormalities than adolescents with bipolar disorder alone in frontal and temporal cortical regions, as well as in subcortical areas linked with emotion and motivational regulation. Although the limited prescan exposure to marijuana in these adolescents tentatively suggests that these findings may reflect underlying differences, the direct effect of cannabis exposure may also be involved.
My opinion is that science doesn't back it. People argue that it's great until they're blue in the face but don't seem to get that they'd bloody well lack the insight to know if it was actually making things better or worse because there's no way in hell for them to be sure they'd BE better or worse without it if they're on it, or on it if they're without it.
There's some promise, especially in high CBD (and low THC) strains, that it could EVENTUALLY be developed into a viable treatment for some people, but, like all other drugs sometimes effective on epilepsy that happen to have some applications for bipolar patients, it probably won't turn out to be a universal solution for everyone.
But the real danger is that drug concentration is hard to get consistent, it could interfere with your doctor-approved treatment regimen (either by masking symptoms so you don't find the holy grail of meds as fast or by making symptoms unintentionally worse so you think a med doesn't work when it does), and, well, it's largely illegal in most places and therefore shady people are the ones distributing and they could very well splice it with shit, or accidentally give you the wrong strain, etc.
And yeah... pretty sure it is what made yours truly go over the deep end and into the hospital bed. Was a very casual dabbler, did a lot more than usual with some trusted friends, bit fuzzy after that but there were cop cars and hospitals, and unpleasantness... 2/10 Do not recommend.