Anything between 16.5 and 18.5 is OK with me, for me. I won't let it get above 18.5.
I have not noticed any physical health problems (apart from being bloody freezing all the time). I don't look emaciated and I eat healthy food, but about 1300 cals a day.
DH disagrees, he worries and thinks I'm "being anorexic" and is concerned about how it might effect my fertility (we are going to start ttc in 6 months).
I think I'm fine and BMI isn't the be all and end all of healthy weights anyway.