The customizable nature of the product. The support for multiple operating systems. This allows me to run a linux server, and still have windows clients. Also the command-line clients allow me to integrate them in a custom build script that can run on multiple platforms.
The only feature missing is the integration of SCM file states and Testtrack workflows.
If you are looking for a good and well integrated SCM / Bug tracking system the combination of Surround SCM and Testtrack is one of the best in the market.
We use Surround SCM for two purposes:
1 Source code management
2 Document Management.
Our source code management use is more traditional use relying heavily on the branching possibilities. We also use it to manage our electronic and mechanical designs.
On the Quality control side we use SCM to store all our documents for our ISO13485 certified QMS. Besides our QMS documents we also use the product to store other design information.