I used to use it on my main machine but stopped for various reasons that were mostly software support related and have now been fixed anyway. Also because it uses the mouse a lot.
Many command line tools work fine. Emacs doesn't work in gui iirc but QT programs work well. There are also a lot using its own toolkit which is much nicer to develop in than any alternative (except for the fact it's C++ which I don't like much). Couldn't really tell you about the license except I'm pretty sure it's free. It might come with driver blobs though. It implements quite a bit of POSIX and depending on what you use your Linux/BSD system for will feel familiar. It's single user though, can only boot into the graphical interface, and feels coherent, which sets it apart.
It's fast. Suspend didn't work, because it hadn't been implemented if I remember right, but that was okay because from an SSD it started in less than 10 seconds. Some people might want things faster than that - my needs were few and I appreciated the gain in battery life (some might argue I was infatuated to the extent of masochism).
It uses keybindings more similar to macos - that is, the alt key is used for copying and pasting. This is a good thing and means copying and pasting works everywhere, even in the terminal without any faff. In a similar no-faff vein, the filesystem is excellent (it's basically a database), easy to navigate and use, and the file selection dialogue and file manager are exactly the same thing; when navigating or manipulating or doing anything with files, you can do something intuitive and it will work.
When I used the system, I tried to fully embrace Haiku life by using the Haiku programs. I used the Haiku IRC client, browser, media player, and email client. The email client by the way grabs emails into files, but instead of being like a maildir file they use the file metadata to store headers, and can be sorted very man of tastely in different ways because of this. There is a focus on file-orientation, like with UNIX, but it seems better thought out here - different to plan 9, and not necessarily better oPost too long. Click here to view the full text.