[maccam94]

There used to be a separate vagrant plugin for rsync but it's now built-in. There is also built-in support for NFS and virtualbox/vmware synced folders. These all work reasonably well until you start having fairly large numbers of files/directories.

Also if you use a native Linux host with LXC or Docker there is no overhead for sharing directories with the container, it's just a bind mount.

[RussianCow]

I don't believe NFS supports inotify events? At least, that's what I'm using, and I'm forced to use polling for any file change detection. And rsync is one-way IIRC. But yes, LXC on Linux works great when it's feasible; I've just been looking for something that supports file change detection on other platforms.