Definitely a spectrum. Look at us here. Some of us have mostly manias while others are stuck in primarily depressive states. Still others function in mixed episodes a majority of the time. The severity of the episodes varies from person to person as well. Some of us report large periods of relative stability and others aren't there yet. Some people with bipolar disorder can work full time, some only part time, others not at all. I do think there's a difference between unipolar depression and bipolar though, so I would disagree with diagnosing someone who had never had any manic or mixed symptoms. Of course, I'm not a doctor, so that's not really my job.