I've been using Pensieve regularly for a few months now. The killer feature for me is being able to search all of my online accounts at once. I can never remember if I put something in Google Drive, or if someone shared it with me in email, or if it was in Slack, or in Evernote, or worse if it's somewhere I rarely go on my own but still have an account like Dropbox or OneDrive. So now I can just hit OPT+SPACE and find what I want right away. And because it's now my go-to place for search, I also now use Pensieve directly for quick notes-to-self, instead of Evernote or Slacks to myself or offline text files. It's just a much better way to keep track of everything.
I'm curious to see where they take it next -- seems like it could go in a lot of different directions from here. But as is, it's already very valuable.