Yes, I have.
It worked the same way shell_exec(), the only "type" of output it provides is related to simple "print" functions.
I took a look at the documentation beforehand and noticed it gathers the output as an array, where each item is a line from the output. Since I couldn't output anything besides simple prints, I tried parsing it into a simple table, with no luck printing "\n" chars.
Regarding trying to output a graph or even a static image, I tried using pipes, which provided me no output when trying to display the item with the child's output (array, if I remember correctly).
Simple prints of the arrays (from exec(), shell_exec(), proc_open(), etc) appear to work fine, anything other than that doesn't.