What package[s] are state-of-the art OR attract you to Julia, and make you stay there (not easily replicateable in e.g. Python, R, MATLAB)?

You would think this, but that fails to be true for large problems (e.g. O(10^6) rows and columns). So you can either have fun finding a more direct way of getting the thing into a standard form, or you have to deal with severe performance limitations of using something like pyomo to do it. Either way, you run into my anecdotes above.