OK, First the positive. I've been playing around with VirtualBox in Ubuntu, using a Vista guest, and loving it. Specifically, it's great to be able to use NAT (Network Address Translation) inside the virtual machine networking setup to access the internet (however, having my router which is currently offline hooked up, online, and using Host networking to actually place the virtual machine on the local LAN goes beyond "great" and comes just a tad shy of "magnificient"). I made a happy discovery that I did not need the guest additions to access my Samba folders on the Ubuntu Linux side, because I can just map a drive to the Samba folder, for example, \\10.0.2.2\D can be mapped to X: on the virtual machine. Sweet.
Now, the negative. Seems like I SHOULD be able to, on the host machine side, see the virtual machine as 10.0.2.15 (the IP address assigned to it by VirtualBox), but for some reason, it only worked for a few brief seconds and then stopped altogether. I have no idea why. In this regard, I preferred VMware because I liked the way it did NAT networking better, but to be honest, that was several versions back.
More positive, now. Kudos to the VirtualBox people for removing the old clunky Linux networking interface and making everything just work. They don't get nearly the praise they deserve for that. I actually clapped for them just now, but you'll have to take my word for it.