I spent a good portion of my time late nineties working with Lotus Notes/Domino. I even had all the certifications (Principal CLP Application Developer, Principal CLP System Administrator). (And I worked on migrations from OfficeVision and from cc:Mail.)
A few things I remember really liking, from both a development and administration perspective:
- trivial to set up a second or third server, and have data automatically replicate between them
- super-easy to create CRUD apps for custom business processes, without using any 'code' except formulae that were as simple as spreadsheet formulae
- ability to add extra functionality with a language very similar to Visual Basic
And a few things that were annoying:
- because email was just another Notes application, it wasn't as good as things made just for email (like Outlook or Eudora)
- many people thought 'views' were folders, and that if an email existed in two places that meant two copies, and that they could delete one of them
- 'replication or save conflict' (https://help.hcl-software.com/dom_designer/14.5.0/basic/H_AB...)
Several times I sat with a user at their desk, and developed the first version of a CRUD app for them in real time, whilst we were still discussing the requirements.