Underslung grenade launchers aren't as cool as you would think. Their only real use is at national OPs or other OPs where there's vehicles that are only destroyable by howlers.
Or close quarters, where a blast from a 203 can deter a team attempting to make entry(sucks to be the first man!).
Like Spazz said, it does add wieght, but if you use it enough, you might find it worth the wieght. I was lucky enough to be lent one for a few hours, and I enjoyed using it, and used it every round, when appropriate.
The biggest downside, is cost. They are expensive, and extra grenades are fairly expensive, especially for something that can't be reloaded effeciently in the field.
