Like others have said, I have seen it work really, really well for the kids but only where the adults are prepared and able to work together really well and collaboratively for the sake of the kids, putting aside whatever and whoever caused the relationship to fail, and properly commit to the kids having a truly equal relationship with both parents, not one being their 'real parent' and 'proper home' and the other being where they visit. This meant:
Both homes being equally set up for the kids, complete sets of everything at both homes i.e. two sets of school uniforms, two PlayStations, two lunchboxes and everything else, only a very few special or comfort items going between the homes so there was never hassle about things they wanted being at the other house or cause for conflict on whose turn it was to iron the school shirts or whatever. One family I know did shuttle the family dog around between both houses as custody of him was shared too!
The two houses being close by, within walking distance so although they started with a strict routine when the kids were littler so they knew who they'd be with which night, this could get gradually more flexible and relaxed as the kids got older and had more activities and things after school, until by the time they were teens they basically floated between whichever house took their fancy.
Both parents being really chilled and non dogmatic about their 'rights' and 'having' to have the kids their exact 50%, if a kid was sick and just wanted mum that day Dad wasn't going to insist they came back to his because it's 'his' day and equally if they were wanting to play a game or watch a film with Dad Sunday night mum wasn't insistent on them being back for 5pm on the dot. Holidays, special occasions were either shared or divided in a really sensitive way and again never ever allowing adult emotions to get in the way of the kids enjoyment.
To be clear I'm not saying everyone can and should parent in this way when they split. There's lots of reasons why it might not be possible and a more uneven division of time is best for the kids, in the short or long term. But I do think it's very wrong to say 50:50 is automatically cruel and unreasonable, I think on the contrary its actually the ideal to be aimed for...