Actually, as with d3, you can develop your own network visualization layouts.
The main difference is that sigma only deals with graphs drawing, so it deals without effort with drag and drop (mouse or touch), scaling the graph to the screen, smooth zooming (with mousewheel or pinch)...
It depends on where your data's coming from, but I tend to avoid doing layout in JS, and instead pre-lay-out the data using Gephi or graphviz (possibly in a cron job if the data changes), loading the hardcoded initial layout in JS and using sigma.js only for display/interaction. In the non-cron case, also lets me tweak the layout a bit through trial and error or trying out different algorithms, to get something I like. Obviously doesn't work for every possible usage, though.
