I use my round pen quite often in the summer--almost daily. Since I do not have an enclosed arena, I use my round pen for lunging, grooming areas, training, separating horses, etc. It is nice to have, especially since it is portable, but it is not very good for riding in--there isn't much that you can do in it. I have been considering setting it up in an 'L' formation in a corner of one of my pastures (secured with posts so it doesn't tip) so I could have an arena.
All in all, I am glad I have one and that it is so versatile. But if you have an arena, I can't see much reason in needing a round pen, unless you wanted to pursue free lunging or needed something that was portable.