There are a couple of reasons why I like the round design. I have a couple of rectangular ovens at home (in/out), and have been disappointed. I have installed round ovens in a couple of houses in Italy, and like them much better (which is why we came up with the plans). There are a couple things I have seen.
1. With a round oven you have room on the side for your fire and food/pizza in the side on the back. It's all reachable. With my 32x36 rectangular oven, there isn't a good place for the fire -- it's basically a one pizza oven. If you put the fire on the side, you have very little room on the side, and the back is lost. If you put it in the back, the fire doesn't reflect to the front of the oven. A 35" round gives you much more usable space than a 32x36, and uses about the same amount of material and footprint.
2. The round, spherical dome does a better job of bouncing heat down to the cooking floor evenly. You can cook pizza everywhere (or roasts and veggies) in the oven, and it cooks evenly. That is how the high volume pizzerias cook all those pizzas. The rectangular oven has a barrel vault, which gives you hot and cool spots, depending where the fire it.
3. The round dome is self-standing (ala the Duomo in Florence), so it does not need a lot of concrete clapping to hold it together. It is lighter and heats up much faster. A round oven will heat in an hour (or less depending on the type), where the heavier ovens take 2-3 hours, or more. That means you are burning more wood (which isn't good for the environment or your pocketbook). For me, the heat time is the difference between using my oven after work, or not.
There are also little things, like clean up.
The downside is that you can only bake around 25 loaves of bread from a single firing, not 75, but for a home oven, that works for me. I would note that all of the oven in Italy are round -- brick and prefabricated.
There are bunch of postings from builders who have seen the same thing on the Yahoo! user group.