I've used this a few times over the years. Pretty useful if you just want to feed a bunch of nodes and edges and get a sense of what's what. I used a python wrapper.

Ancient software that works but you can feel its age.

It would be great if someone would create a fork implementing a modern syntax and implemented as modules for multiple platforms.

The syntax is actually still a very nice way of quickly writing down a graph. However, graphviz mixes structural information (nodes, edges) with style information (color, shape), and layout information (preferred rank assignment, dummy nodes and edges to make the algorithm do what you want, or other hacks such as edges that should not be considered directed). This makes it difficult to tweak the layout and also bloats the format with very many things that are more or less irrelevant if you're just after the graph structure.

We've recently added very barebones Graphviz import to our online graph drawing application and I've compiled an internal list of features that could be supported eventually and which will never be. There are a few quite annoying behaviors and corner cases in some attributes that make re-implementing them elsewhere difficult.