Periodically (read: every day), I wonder about switching from Domino to a SQL server for data storage on my guild web site. The primary reason for this is speed: I'm doing primarily relational things, so I've had to wrangle Domino quite a bit to do this with any amount of speed and code cleanliness. Additionally, while most of my documents are entirely distinct from each other, I had to make concessions here and there, such as storing the latest Post date in Topic documents so I can sort them that way, and, each time I have to do that, there's another little bit of code maintenance and clustering-unsafety.
However, my ideas always come to a screeching halt when I remember Reader fields. They're simply too good, and the replacements I've found on the open source SQL databases have been, to put it kindly, lacking in comparison. They generally involve having some access level field or, best case, a multi-value field of names that are allowed to see the document, and then making sure that all of your queries or views honor that. Every method has some severe downside, ranging from inflexibility (access level) to nightmarish piles of code everywhere (multi-value name/group/role fields). Everywhere I accessed the database, I'd have to worry about security and document access, bloating up the code and just asking for data-leak bugs.
Domino, for all of its faults, makes this something you just don't have to worry about. If you have a Reader field, you can toss names, groups, and roles in there with impunity, and the server will handle the rest like you'd want. You don't have to do your own directory lookups, security checks, or nested queries. If the current user isn't on the list, the document may as well not exist. Even if the user had the UNID of the document and designer access to the database, it'd be beyond their reach. This is enormously comforting. And even though it's just a guild web site and not a giant corporate database, I'd still rather deal with a bit of tricky code for performance than the headaches and drama involved with people seeing what they're not allowed to see.
So, until I either get entirely fed up with Designer or I find an equivalent to Reader fields in a free SQL server, I'll be sticking with Domino.