My girlfriend suffers from bipolar type II. With medication it's manageable, though she can be difficult to control when she's manic and hard to be around when she's depressed. She tries not to let it get in the way of our relationship. So, all in all, not too bad.

On the other hand, my mother has an exhaustion and maladaptation disorder after severe stress that resulted in a total burnout of her nervous system. She has [i]no[/i] energy reserves, suffers from heavy hyperacusis and is unable to handle a heavy emotional load. Her life is basically ruined. She was diagnosed 11 years ago and is not expected to ever recover. 

That's my experience with mental illness, though my mother's disorder might be more neurological than anything.