Sadly, it's not great. There's the OS compatibility issue someone described above, but also, both Apple's and Nvidia's drivers are very glitchy. I've tried 650Ti, 660, and 1060 on my Mac Pro. Sure they're faster than the original cards but way slower than they should be and prone to crashes, screen artifacts, etc. Though they work fine in Windows under Boot Camp. There's also technically CUDA support, but it's limited and painful. I've tried setting up both TensorFlow and PyTorch in macOS, went pretty deep in and gave up.