I had to use Notes after our company was acquired by IBM in 2005. As a startup, we used Thunderbird. What I remember was that finding things in Thunderbird was just so much faster because it had a filter feature (you enter a string, then instantly only the messages with that string would be shown). No such feature in Notes- I remember having a hard time finding messages, and that its search feature would miss things somehow.

Yeah, Notes had its shared database features, and you could see remnants of IBMers using them at the time, but most such applications been ported to Java CRUD apps. Also Lotus Sametime (it's like Slack) became a much more important collaboration tool.