As someone who has hereditary problems and diseases and such running in my family, I would fully support the usage of genetic engineering in disease prevention and making sure the baby is healthy.
I can see why appearance modifications may be used too, because honestly, no one wants an ugly child, and no one -wants- to be ugly. And maybe you have several sons and want a daughter so you use genetic engineering to control the gender of your baby, which I think is acceptable. However, I also think that just making your baby pretty because you want a pretty baby is kind of shallow. I can see both sides here.
As for behavioral modifications, this can be a double-edged sword. I can understand that a parent would want to get rid of "bad" behaviors in their child, because no one wants to be the parent of a gangster or a thief or a murderer and behavioral modifications can help prevent behaviors that would develop into these kinds of mindsets. However, changing behaviors because you want your child in real life to be exactly how you imagine it to be... While I can understand this desire to have children with personalities and such that you would enjoy being around and be able to get along with, that would be where I draw the line. As someone said before, this brings up issues of free will, etc., which shouldn't be dabbled in.
Bottom line: it would probably be better if genetic engineering went untouched, but that is an impossibility now, so rules need to be set in place, and soon, to prevent its abuse and a world like the movie Gattaca, which someone linked to.