I’ve recently completed my first custom tool for CadCorp’s Map Modeller. CadCorp is most used in the UK, and it is apparently also big in Japan. The main benefit is that it reads many different spatial data formats directly, it can connect to SQL Server and Oracle without middleware (i.e. an SDE type application), and it is a leader in the open standards espoused by the Open Geospatial Consortium.
In terms of development the main issue is that there appear to be very few people developing tools around CadCorp’s products apart from CadCorp themselves. This means that the only code samples are in the help system installed with the tool, and a PDF file. Straying outside these samples I found involves a lot of trial and error. There is a forum on the CadCorp site – annoyingly registration is required to read the posts, and there are very few programming related posts. The CadCorp staff do however get back to you on any questions you pose.