I've thought about this long and hard. Not nearly long and hard enough, though.
My position right now? I live in Canada, which is at replacement rate. We aren't overpopulated - Other countries are (China, India...)
I don't feel, personally, that having a child of my own is really impacting the problem of overpopulation too much, because I know that my child will be cared for and have access to family planning, and it's unlikely that my kid will have 24 children of their own.
Adopting helps. Not having children helps. But the root of the problem is in poor government care. It's been statistically proven that women who have access to education and family planning will marry later and have fewer children - Regardless of religious belief or cultural values.
Note, fewer. For some, that might mean having three children instead of seven.
I would be more worried about the environmental impact than the increasing population, personally, because I know that my country's population has reached a plateau.
My country consumes far more resources than "less developed countries". More resources per person.
I wouldn't feel bad for having a kid, but I would put a HUGE stress on the importance of sustainable living. Preach it to my child so that their carbon footprint is always on their mind.
In any case, I'm in no state to be a parent right now and won't be for many, many years.
So, when the time comes, I'll have had plenty of time to decide whether it's worth it or not. xP Right now I have bigger fish to fry.