I am one of those fellows that tend to run on the trunk.
What does it mean? It means that I am using the latest version of the source for my projects. So, I am on the trunk for Boo, NHibernate and Castle (which can be tricky, since Castle isn't on NHibernate trunk).
This has significant advantages for an active OSS project, NHibernate's trunk has the multi criteria support, MonoRail's trunk has the testing support, and since stuff is moving in response to the needs of the users, it is evolving fast. Sean Chambers was impressed by how fast this can happen in many cases. (In others, it is a slower process, but still mostly visible.)
There are some disadvantages, people (recently it was mostly me, sadly) can break the trunk, or commit functionality that breaks your code. In general, I have not found this to be the case for the projects that I am working on. A lot of the committers are running on the trunk as well, and there is a big pressure to make sure that the trunk is stable (ready to ship) because I, at least, do ship from it.
The major disadvantage here, from my point of view, is using a piece of code that is still under development or review. The recent case was Criteria cloning in NHibernate, which has gone through some modifications to get to its current form. Personally, I don't find it onerous to do so, and those are usually rare.
The big advantage is that you get continiously improvements. I does reviews of the changes every now again (now limited to scanning the logs and looking at what has changed) and it keep amaze me how much good stuff is going in there.