Since we didn't need a reservation, the most important thing for getting a campsite is getting up early. First come first serve at the South site means lots of people in line. We got to the campground at around 6:30 on a Sunday, because we wanted to be sure to get a spot. That early in the morning, we were already behind about 12-15 cars/RVs. Since they start assigning spots at 7:00-ish, I would strongly recommend getting there early. We had no trouble getting a spot, but I would still say better safe than sorry! If you can't/don't want to get there that early, you may still be okay. Later in the day several more campsites opened up next to our spot and weren't filled for a few hours at least. (Checkout is around 11:00 if you want to plan for getting there later).
We had a larger group with us, so the site we had was pretty large, and we had room to spare! For a regular site (not the group sites) you can have up to 6 people, 2 cars, and 3 tents. The group sites allow more people, but there are fewer of those available. Keep that in mind if you have an even larger party. Side by side sites could also be a possibility, but getting there early would be important.
The site itself was well kept and pretty quiet. I didn't take any pictures of the site, but the views all around it were beautiful! Just a short walk from the campground is the Visitor Center. Here you can access the shuttle bus, which takes you down the canyon. The shuttle takes you to all the trails and interesting locations.
A word of warning: The shuttle bus stop at the visitor's center is like a line for a ride at Disney world. It is CRAZY! I was shocked by the amount of people there, especially since I went at the end of March. If you're going in summer, I imagine it would only be worse. If you want to avoid this line (which I waited in once for over 45 mins) then I have some tips:
Overall, camping at the South site was convenient and beautiful. I was not much for all the people that were there, but it's no wonder why they all come; Zion is just too beautiful. Since you can work around all the people, I'd say the views are definitely worth it.