We've been using Sandbox at Stroud College in Gloucestershire, UK for a few months now. We run a two year full-time Games Development Course for 16-19 year olds, and have been using Sandbox as a gentle introduction to 3D game design principles for new students. The students like it because the learning curve is so easy, and they take to it very quickly. Although many of them eventually move on to more "professional" tools (given the age group and focus of the course), it still forms a very useful first step that gives them confidence in their ability to design and prototype game levels.
You can see some basic details about the course here:
http://stroud.ac.uk/btec-national-diplo ... multimediaObligatory marketing stuff about the college:
Stroud College’s School of Art & Design is regarded as one of the best in the country due to our superb state of the art facilities and professional, experienced staff. We have excellent facilities for studying including professional studios, workshops, design suites, specialist ceramics studios and an outstanding photography studio with a spacious darkroom.
All our teaching rooms are installed with high tech, modern equipment and software to deliver a first class teaching and learning experience.
Our excellence in this area has been recognised in past Ofsted inspection reports, where the department gained a Grade 1 for outstanding provision, ranking it as among the best in the country.