10+ years ago I'd regularly build all sorts of little utilities with it. It was surprisingly easy to use it to tap into things that are otherwise a lot more work. For instance I used it to monitor the data coming from a USB device. Like 3 nodes and 3 patches to make all of that work. Working little GUI app in seconds.
Apple hasn't touched it since 2016, I kind of hope it makes a comeback given Blender and more so Unreal Engine giving people a taste of the node based visual programming life.
You can still download it from Apple, and it still technically works but a lot of the most powerful nodes are broken in the newer OS's. I'd love to see the whole thing revitalized.