In the right environment, co-ed soccer is a great one for a young players' development--both socially and in terms of skill learning. You do have to bear in mind, however, that boys and girls develop at different rates, both physically and socially. Early on in years, girls develop faster and this can help offset the physical advantages boys usually have over girls.

As the boys and girls both grow older and enter adolescence, however, differences between the two genders are bigger, and I would encourage same-gender teams when possible. In the younger ages, co-ed soccer is perfectly fine for player development--even though the boys may think the girls have "cooties"!