I find that being able to sketch up things I need in CAD and then print them is both cool and really useful. It's the main reason I bought a printer in the first place. Thus far I've tried out FreeCAD, Solid Edge, and Blender. With any modeling package, you will have to dedicate time on a regular basis to really get used to them. FreeCAD is certainly nice for the fact it's free, just as it implies. I used it to design a few parts that were functional. It works, has some useful workbenches and add-ons. My problems were the software having bugs that caused models to break when trying to make changes, and available training info was often outdated. Siemens offers a free version of Solid Edge to makers, which is really nice, even with some of the advanced features turned off. It's a much more polished program with great training resources. You can only export designs as stl files, but that's fine for 3d printing. Solid Edge will slice and print, but I always import files into Orca and go from there. Blender looks really amazing for modeling, but I admit I haven't spent enough time learning it yet. You can use it to manipulate meshes, which is useful for customizing and fixing models. I've used it to Frankenstein together different models for custom prints I wanted. But yeah, while you don't have to learn to use modeling software to do prints, it opens up so many options for you to be creative. I think it's worth while.