Other features
Does an application have a worthwhile extra that others lack?
Ipe has a lot of facilities aimed at users of the LaTeX text layout language that is often used in academic work. If you know how to work with this language, you can do things such as add complex equations directly into a diagram, and Ipe has facilities to make sure that the fonts in your diagram are consistent with the fonts in your document. It also has a stylesheet capability, so you can set attributes such as fonts, colours or line types and carry them over between drawings.