I have a 50/50 split with my DH. We do the more complicated 3/4 day split swapping on Wednesdays and Sundays. It works really well for DS. We were concerned that it might be complicated and unsettling, but actually it means that he is not without either of us for too long. DS is at a special school, so is picked up for school everyday by taxi from his father's house, so he always gets an hour with his df in the mornings regardless of who's day it is. We live just round the corner from each other so if he needs one of us we can always be there.
One of the benefits of splitting his time the way we do is that, rather than - as one poster suggested above - mummy does swimming club and daddy does speech therapy, or whatever, we both do everything so don't miss being able to talk (or sign) to him about the things he does.
This, however, only works because we are still extremely good friends, our split has always been amicable and we both adore our ds and would do anything to make it easier for him.
If things change as he gets older and wants one room in one house, then we will accommodate that and it will be up to him (not in a 'choose a parent' kind of way. We will both be understanding and supportive).
As inveterate says above, though, I would worry that this would be an extremely difficult arrangement if you don't get on with one another.