Probably like ADHD, lots of docs would suggest that bipolar is both over- and under-diagnosed. With the rise of the atypical antipsychotics, it became sort of a 'flavor of the month' diagnosis for folks with anxious depression, or treatment-resistant depression, or irritable depression, or depression with substance use disorders. On the other hand - it's still underdiagnosed as well, with one (older) survey suggesting ~1/3 of folks waited a decade or more for a correct diagnosis [0], and no reason to believe this has improved all that much.

[0] https://www.ncbi.nlm.nih.gov/pubmed/12633125