I look forward to one day getting (or building) a wood splitter, but right now, with two teenage boys, I just can't justify it. In fact it would probably slow our wood processing system down. Practically everyone agrees that you can split wood manually faster than you can split it with a hydraulic splitter, but only for a while, then you get tired and slow down, where with a hydraulic splitter you can keep up a stead pace without getting tired. With our system we operate tag team on the splitting maul, chop like heck on the maul until you get tired, then trade off and stack for a while until you get the urge to split again. Using this system we can go through a cord or two in very quick order.
The benefits of manual splitting are great and the confidence and skill my boys have gained with that splitting maul is impressive, few adult men can wield that maul like they can. One other benefit my older son has with manual splitting is that he is able apply his wood splitting time towards his physical education schooling (he takes home schooling).
All that said I would still like to get a hydraulic splitter, because they are great power tools to play around with and I'm always thinking about what type and size would be best for us and how we would use it, and I've been following this vertical/horizontal debate to some degree. The way I see it, I would probably be splitting most of my stuff horizontal and here is why.
All of us try our best to move the wood the least amount of times possible. In my case I go cut and haul my own wood home in my pick up. Then we usually unload the rounds in a pile near our splitting area close to the wood shed. From there we split it and stack it in the shed. Because we cut standing dead wood only it is almost always ready to burn, but even if it needs a little more seasoning time my shed is long and narrow, so the wood gets plenty of sun and air to dry while it's in the shed. If I had a hydraulic splitter it would save a step and be much easier to split the wood as it comes off the back of the truck, and from there it would go directly into the wood shed. This would eliminate a whole wood processing step and reduce the amount of bending over quite a bit.
Of course not everybody has the same system, and this wouldn't work for a lot of wood burners, but the way I see it happening you can count us in among those who support horizontal splitting, if and when we ever get a hydraulic splitter that is. ;-P