My ex and I share custody of our son 50/50. Our son is now 10, and we've been apart since he was 9 months old. Obviously, we didn't start splitting custody 50/50 immediately, as at the time our son was still nursing far too much to be able to do that. But it's been since he was around 3 or 4 that we've been doing this. His father moved closer to us when he was about 4, and that made a huge difference. He's now about a 5 minute walk away, which means our son is even on the same bus route for school (though now he's going to a charter school in another district which means he gets driven back and forth). It's working very well. Our son is very happy and well adjusted, and has wonderful relationships with both families, is very close to all of his siblings (he has 2 little siblings in each household.) He spends Mon and Tues nights at his dad's, Wed and Thurs with us, and F, S, and Su alternate between the 2 households. With a situation like this, you need to be very very organized, and willing to adapt and change the schedule as your child grows and his needs change. When he was younger, our son wasn't able to handle the longer stretches at one or the other house that he can now -- the longest he went without seeing the other family was 2 days. Now it's up to 5. Younger children need to connect with the other parent more frequently in order to maintain that relationship. It's also much easier, scheduling-wise, to always have the same days of the week -- that way, for instance, you always take him to soccer and your ex always takes him to cub scouts. That way you have a consistent schedule and the required things for each activity don't need to go back and forth.
You didn't ask about this, but I'm going to talk about it anyway -- since we split custody and both maintain clothes, toys, etc, and spend equally on housing and food, we don't do child support. Instead, we each contribute equally on a monthly basis to a joint account which covers our son's expenses (other than room and board -- we assume those are roughly equal, and any special activities/vacations that he does with either family) like clothes, school supplies, and extracurriculars, his share of health insurance, and other things that we agree upon. We've worked very hard to be able to co-parent together successfully, and it isn't always easy, but I think it truly is the best thing for our son.