I use it regularly. Mostly for visualization and prototyping ideas. I’ve tried over the years to replace it with Python/Sage, but the giant library of builtin functions always pulls me back. Plus, the lisp-ish language I prefer over Python. The visualization capabilities are quite nice too - not just data or function plots, but the ability to construct relatively complex graphics programmatically is quite useful. On a recent project where I needed to build some data structures for spatial processing in 3d, I was able to build a nice 3d visualization to help me debug my code by visualizing the calculations I was doing. I also really like the backwards compatibility of notebooks: I have mathematica notebooks dating back to the 90s, many of which still work fine (modulo some visual ugliness due to changes they made to layout and presentation over the decades). Few other tools have that longevity.