Group Psychotherapy is, in my experience, the most intensive and effective modality of psychotherapy. The group has the potential to become a microcosm of the real world, and the learning opportunities multiply exponentially when people are looking at how they relate to others in the present moment.
Group Psychotherapy isn't more widely used in part because few clinicians are trained in it, and the few who do run groups often use it as an "advice-giving" session or go around the group doing "one-on-one" therapy--which completely misses the point. Few people comprehend how powerful it can be. It takes someone specifically skilled in facilitating group process and leveraging what arises in the "here-and-now" for its members to take full advantage of the opportunities group therapy offers.