I'm told it is a pretty cable structural system, I know of at least one engineer in my area who uses it for mixed material custom homes and commercial projects. He swears by it.
I have attempted to dive in and get it working for some practical problems, and some more complex problems. With varying degrees of success.
My biggest complaint is that the software is structured in such an unfamiliar way. It is very difficult to understand how the UI works. It is kind of set up like ram structural system, where you modify your model geometry, sections, loading, and supports via different modules. It feels like each of these modules were created by different folks and it isn't always clear which module you should be in.
I have found that for what seems like a relatively simple task, Robot consumes alot of time just navigating the UI and figuring out what tool to use.
Setting up things like design section sets, design rules is possible, but fraught with difficulty.
Getting seemingly simple results from your analysis is also very complex.
Lastly, I have found it to be somewhat unstable. It crashes more than other programs I use, often with no error message or recovery.
But if you can get over the hump of learning the structure of the UI and get some nice templates setup I think it could be really useful and for the value you really couldn't beat it if you have Revit already.
Last time I used it was to do a rigid diaphragm analysis on a slab with significant torsional irregularity. I could have probably done it in Risa3d in 1/3rd the time.