Just to offer an alternative angle to a lot of the stories I've read on here, my dad has pretty severe bipolar disorder. He had a major collapse while, somewhat ironically but also perhaps not surprisingly, working as a psychiatric nurse at the royal bethlem. I still remember visiting him in the in patients ward at our local hospital, even though I was quite young at the time, and I still remember how small and sad he seemed.
However.
Ever since he's been on daily medication, and up until a few years ago regular visits to the psychiatrist (as in every 2-3 months), and honestly to meet him you would not have a clue. He is a fully functioning, witty, caring, lovely person, albeit with a hilariously inappropriate sense of humour. Both of my parents are like that though, so I doubt that is entirely the effect of the mania.
He is on medication which maintains him at a happy medium, but doesn't restrict him, and to see him in full flight is still rather impressive, you can just see his brain fizzing. In short, he is lovely to be around. Of course, that can be part of the tragedy of the manic depressive, they're often very charismatic, lively people, until the low hits, but he is in control, and he is happy, and honestly I sometimes have to remind myself that he has a mental illness at all. And it isn't a dirty family secret either, my parents even make jokes about it. I know that if I ever have any questions about his disorder I can ask him and he will answer as honestly as he can, so there is never any 'black dog' hanging over the family, just a loving, caring, if slightly crazy, father and husband.
Just thought I'd offer that as another point of view - obviously it's only one particular kind of disorder and others will be associated with other experiences I'm sure, but still. Make of it what you will (: