I recently installed VirtualBox Guest Additions to an Ubuntu Server 8.04.1 VM that's running the -virtual kernel mentioned in my last post.
It was a breeze. The installer told me specifically what it couldn't find when necessary (make, gcc, kernel headers), didn't barf with crazy error messages, didn't spew pages and pages of unneeded details about how the kernel module compilation was proceeding when it was working properly, and it Just Worked the first time on a fully-updated machine. On reboot of the VM, the kernel modules loaded and worked exactly as expected.
Contrast this to VMware Tools, which is hit or miss on the "will it work the first time" front, especially when installing on a guest OS that's newer than the Tools are designed for. It also spews files all over the place, and looking at the scripts gives me a headache, especially compared to the simplicity of the VirtualBox scripts.
I'm not sure if the elegance and simplicity of the VirtualBox Additions install and operation can be attributed to the original creators of the software, or to improvements since Sun purchased the company, or a bit of both, but regardless I like it.
Also, I've been doing some really intensive I/O and CPU work (transcoding DivX videos into MPEG2 using ffmpeg) inside this VirtualBox VM running on a Windows XP host, reading/writing files on a Shared Folder from the host, and both the guest and host have been very fast and entirely stable. I'd say the transcoding is at close to native speed, though that's entirely subjective and without scientific evidence.
If VirtualBox could just get FreeBSD support working, I'd have no complaints left.