I use pallets for a lot of things- temporary fences is one of them. I use chicken wire to line the bottoms because we have nigis and mini nubians... babies are tiny enough to fit through many of the pallets.
They are short at only 4 feet. I have one mini nubian that I'm sure will one day "accidentally" take a running leap over and then I'll have to run a strand of hot wire at the top.
I think if you find enough of them to be fairly uniform in spacing, they *can* look really cute. You could even paint them
I like that my pyr can stand up and look over the top of them easily... I'm sure she'd jump right over if there was something nasty out there. I also like that they have the little "shelf" at the top, so it makes a good place for setting a bucket when I need to open the gate and get through. They are easy enough for me to lift things over, set them down, etc.
I have seen them done very poorly as well... jankety and tied together with bailing twine. Falling into the ground and looking like a dump yard.
If you can find a store in the city that gets deliveries on pallets, they'll LOVE for you to take them away so they don't have to pay to throw them away. And the pallets will most likely be very clean and more uniform looking than what you'd get from a feed store, etc.
They are GREAT for fencing pigs, (at least our AGH) as long as they are anchored with sturdy tposts and if lined with hot wire they never touch them.