For the record, I'm a clinical social worker, so I own my bias.
I think N2N is a very solid depiction of mental health issues and how they impact a family system. I believe they do very well under the constraints of trying to entertain, be under 3 hours, etc.
Actually, I walked away from the show thinking it was a show about grief and loss issues and not really about bi-polar disorder at all. I kind of felt like "bi-polar" which is in all the press notes and materials about the musical is really more of a smoke screen than truly what Diana suffers from.
Diana presents as a puzzle that gets labeled as bi-polar with delusions. The complex truth is that she misses her son and everything she knew, including herself and her husband, that changed after his death.
She probably has manic episodes as evidenced in the opening number with the sandwiches, but I really love the line where she sings, "what happens if the cut, the burn, the break, was never in my brain or blood, but in my soul." That spoke volumes to me about her journey. She was rushed out of her grief by a type-A husband who quickly got her pregnant and tried to erase her grief. The implications from that are huge.
So I really see it as a show about grief and loss more than anything else.
I also like the theories I have heard that since Gabe died as a baby his incarnation is Dan at the age when Gabe died, making the end more poignant as Dan makes peace with his younger self that tried to brush over the past without grieving it.
"The sexual energy between the mother and son really concerns me!"-random woman behind me at Next to Normal
"I want to meet him after and bang him!"-random woman who exposed her breasts at Rock of Ages, referring to James Carpinello