They're popular in some domains. Aside from the ones already mentioned in this thread, there's Softimage ICE which I understand lives on in other forms in 3D m

They're popular in some domains. Aside from the ones already mentioned in this thread, there's Softimage ICE which I understand lives on in other forms in 3D modeling tools. I worked on a visual dataflow language for complex ETL between databases some time ago. My experience with that was I would rather write Scheme directly (which is basically what the pretty directed graphs turned into). Mostly out of familiarity, but I did find the textual representation was denser and so faster to work with once familiar with it; maybe it's all about having the right level of abstraction for the task at hand. I suspect a successful general purpose visual programming language would have to have some really powerful abstractions for parallelism and concurrency to take off in place of languages that already exist. You also have to remember the problems people have had integrating Smalltalk's image-based persistence with non-Smalltalk parts of a system, and avoid that kind of insularity.