There are plenty of thing about Source Insight that ilked
1) Syntax Formatting : It makes code look pretty interesting and it has multiple colors but never looks cluttered at all
2) reference/call tree /call graph: It is a very useful feature which automatically check and identify the reference for what user is writing or where cursor is placed.
3) symbol window : This is again useful in terms to get all the symbols of a file listed.
4) context window : This is also a very interesting Feature of source Insight which automatically populate contextual information of symbol under cursor.
5) Quickest Naviagtion : This helps save developer lot of time while doing their coding
6) Relation Window : This help to figure out relation of a particular symbol with rest of the symbols
7) Clip window : This is also very useful feature of source Insight which helps developer editing like drag and drop functionality of text between from clip to source files.
8) Debugging is also pretty useful feature of source Insight
9) Multiple independent source window which makes it very easy to manage symbol of all files separately in their separate symbol window. Review collected by and hosted on G2.com.