When I initially signed up for Notion, I did so thinking the API was already usable. It was used as a selling point for their premium service even though it wasn't implemented. This was a year and a half ago or so. I felt pretty burned by it and killed my subscription. Hopefully this turns out well for the people who've been waiting for it, but that experience left a terrible taste in my mouth.
That said, Notion is a really good product.