I use it solely for web developement and testing exlporer and other browsers on windows, and was actually able to get Windows to access my lamp server running locally on Ubuntu by using NAT and modifying the hosts file to point my named virtual hosts to 10.0.2.2.
So if anyone has being dual-booting for the same purpose, or using VMware, this is an excellent alternative.
I have to say I'm pretty impressed with VirtualBox so far. I've had all sorts of trouble with Xen in the past, so I'm slowly phasing those VMs out in favour of VirtualBox. I prefer Xen conceptually, but it's just too damn flaky for the real world.
One VirtualBox tip: using virtualisation instructions on recent CPUs seems to be off by default, and it's worth enabling for the speedup if you've got the hardware.
As for networking, I'm using bridging, (see e.g.: [http://www.virtualbox.org/wiki/Advanced_Networking_Linux]) so the virtual machine is an equal peer in my LAN.
I didnt got the 'seamless' route suggested in the article, instead I only went as far as installing the GuestAdditions which solved a few minor nitpicks, and everything is perfect now.