I've tried this, actually. It didn't work for me. I love having a good, brothy soup; I especially like anything with a good chicken broth. And it worked for a few days, sure. Then it got. . .dull. Boring.
Soup isn't convenient. This is good when it comes to stopping mindless eating. It isn't good for when you are dead tired, and running late, and you
need to be out the door in ten minutes. If you can make sure that you're going to have the chance to sit down and eat all your meals, that might not be an issue for you. I also found that, for me, soups were tied to bread; I want my soup with buttered bread, dipped in and soaked in umami-goodness. This isn't exactly ideal for calorie counting

.
That being said, soup is great, and if it works for you, it works for you. I've found it to be most helpful when I'm going out somewhere without calorie counts. A clear broth soup will fill me up a bit, and the calorie count is usually not terrible. It's especially useful when I go out for sushi--a bit of miso soup will sate me and keep me from pigging out on the rolls and nigiri.