It gets called "groupware" nowadays, but even that term needs explaining. When reading below, remember that I administered a small Domino install for about 8 months - just admin. I didn't develop nor did we have a lot of Notes programs.

Notes is really an environment - the client functions a little like an X Server in it's own window except with a different communication protocol and unique standard widgets. Applications run on the server for you but present their information in the Notes client. The server also runs client-less tasks, like the SMTP process. It was designed to make it easy to do workflow (which explains why it does email) but in the last 5 years or so Lotus have made it easier to do HTML stuff, as well. The email client has improved from that depicted in the Interface Hall of Shame, but it could still be better. A number of the problems cited on that site stem from the way Notes does things - not the email client. Once you see that in a broader context, most of those things make more sense.

From a practical viewpoint, Notes clients are Windows only. Wait - there might be a Mac client. There is no Linux client and last time I checked Lotus were showing no interest in creating one. The Windows Domino server is very stable and it doesn't hog the server like Exchange does - i.e. you can run things like file serving on it at the same time. The Linux server took a long time to get stable, but I think it's fine now (learn about GNU screen if you want to run Domino on Linux - it is a godsend).

Server-to-server communication is very good. Replication is highly configurable for making sure things are kept up to date. There are plenty of tools for managing databases, including some really handy ones for invisibly moving them between servers, for instance.

Wade.