I bought a couple of large square bales of hay last time I was at the local feed store to try them out. They can be a disaster! If you have a tractor that can lift them and enough cattle to put them in the field with little waste before they get too wasted, they would be great. The big advantages, IMO, are that they stack really well, and are very large. The biggest disadvantage is that if you store them in the barn and feed portions at a time, they completely fall apart when you cut the strings.
I ended up building a carrier for the tractor out of a pallet and some scrap lumber that will allow me to carry a couple of flakes out to the cows using the pallet forks. It works really well so far. I also used a bungee strap and a couple of hay hooks over the top of the bale to hold the bale together and allow me to release a couple of flakes at a time instead of the entire bale falling apart.
Here is the contraption on the pallet forks, with a couple of hungry dexters eating from it. Cascades Contender looks like he wants to climb in and go for a ride.
Update: I recently bought a bunch of large square bales and have been using my round bale cradle feeder to feed them. I attached a section of hog panel to the base of the cradle feeder to reduce waste, and I am getting 4 days per bale, if I move the herd each day. I have 12 head of cattle right now.
The square bale does not fit initially, but I when I cut the strings and pull a little bit off each end, it falls into the feeder.