Advantages:
Pupils in Co-Ed schools generally have a broader outlook on life as they have been exposed to both male and female views on life and organisation etc.
They know how to behave around the opposite sex and seem to respect them more. Pupils from a single sex school often get nervous and shy around members of the opposite sex as they have no experience of being around them on a daily basis.
Disadvantages:
Co-Ed students can get very distracted by members of the opposite sex, and it may have a detrimental effect on their education. However this also applies to gay or bisexual students in single sex schools.
In my experience, students from single sex schools often seem to be out of touch with life generally and very focused on their education (which is no bad thing in itself) but as soon as they are out of full time education they tend to stay out of touch with life and do not have the skills necessary to converse and relate with members of the opposite sex.