Yes, people with bipolar disorder are often first misdiagnosed, most often with depression. A study a few years back found that, on average, it took individuals 9 years to get an accurate diagnosis. Unfortunately, being misdiagnosed usually means the bipolar disorder is not properly medicated, which can lead to additional problems (for example, if a person diagnosed with bipolar disorder is placed on an anti-depressant without also being on a mood-stabilizing drug, the anti-depressant can sometimes cause a switch into a manic episode; anti-depressants are also just less well-tolerated by people with bipolar disorder, sometimes causing an increase in depression, agitation and anxiety).
As well, of course, the longer the illness goes without being properly treated medically, the more episodes an individual will have, which causes more turmoil and difficulties in the person's life.
However, given the nature of bipolar disorder, it's not a surprise that it is more difficult to diagnose than many of the other mental illnesses. When people are experiencing hypomanic episodes, they feel good; why would they go see their doctor if they feel good?? When their depression gets bad enough, most people will seek medical help, but even then most people who aren't educated in psychiatry will only talk about the symptoms of depression. So it's really important that doctors be on the look-out for symptoms of bipolar disorder when people present with depression, in order to reduce the amount of time to diagnosis. This can make a huge difference in terms of the outcome for people with bipolar disorder - the sooner they receive an accurate diagnosis, the sooner they can get on the right medications and start learning about their illness and how to live with it.