Having been a TabNine user for a while I can say that it's less of a productivity boost and more of a quality of life win. It's hard to measure - not because it's small, but because it's lots of tiny wins, or wins at just the right moment. It makes me happy, and that's why I pay for it - the fact that it's probably also saving me some time is second to the fact that it saves me from annoying interrupts.