We just got one of these at the barn I board at. While I appreciate the practicality of it, it (to me) always looks temporary, so I'm undecided if I want this style at home.
That being said, with a helper it was super easy to set up, and there are some temporary pens set up out of these panels too. Theyre incredibly handy to have around if you're the type that needs to change your set up on occasion.
I love that they're fairly safe and the horses cant chew them to bits, and i like that the panels can be put in almost any configuration.
As far as usuing them as a round pen, Ive had an easier time starting greenies in a solid walled pen-less for them to get distracted by!

Pros: super versatile

Cons: heavy to move

We have an arena we use for most of our schooling, but we also have a round pen. We find it very useful for youngsters and spooky horses. When working with a horse that has been abused, it is very difficult and can be dangerous to use a lunge line or other restraint. The horse learns voice commands faster, I believe, when we use our round pen. We use a 60 ft round pen. It is large enough to stay clear of a bucking horse and small enough, that they can't 'run off'. This is a great training tool if you know how to use one. Ours has held up well over the years and is quite sturdy. We have also used it as a temporary turn out.

Pros: Easy to use

Cons: None

I can't imagine a more useful piece of equipment. You can adjust the size to fit your use as the panels attach very easily together. It is useful for training or for confining a horse if needed. Mine is galvanized steel and is many years old. It is starting to rust around the welds, but will still be useful for years. I think every horse owner would be wise to make use of a round pen.

Pros: Adjustable Size

Cons: Rusts Eventually

I have never used a roundpen until we started fostering rescue horses. I have always used a corner of a pasture and they learned very quickly. The rescue horses are different. They are so fearful, that you really need a confined area to work them. They cannot run away from you, and they learn to bend. Alot easier to manage their speed, and to get them used to voice commands.

Pros: More control

Cons: none

There are many types and sizes of round pens. Do your research carefully. Use one that is safe and large enough for your tasks. I have found the use of a round pen in many ways. We have used it for keeping horses separate, for training, for teaching young riders basics, for safety and for other uses. We have a round pen made of fabric electric fencing. The poles are covered so you aren't able to get hurt from the top.

If used correctly, these are a must have.

Pros: training tool

Cons: need large area