The biggest issue is how easily things switch from "you're perfect" to feeling like you can't do anything right.
No matter how hard you try, how much you love them, there are times that they are impulsive and dangerous, and you can't really help.
If given the chance to do it again, I probably would, but I'd want to take with me the knowledge I've gained from it.
But the bipolar person's partner will always have to do a bit more heavy lifting.
The balance of emotional labor will never be 50/50 - its something that needs to be accepted going into the relationship. I have absolutely no patience for other Bipolar individuals who don't stick to a care plan or medications. For every "batshit crazy" unmedicated/unmanaged bipolar person there is a successful businessman or engineer who just happens to have another aspect of their health that they manage (it isn't easy, and it isn't normal, but that's not the point).
I would think there would be a HUGE difference between someone getting the right medication/treatment for Bipolar, or those who have learned good coping skills, and those who are untreated or undiagnosed.
I have OCD (fairly severe, when unmedicated), but thanks to therapy (years and years ago) and Prozac I can live completely normally.
You can still love someone who is bipolar and have a great relationship, but it requires flexibility and forgiveness.