A while back I blogged about testing VirtualBox. At the same time we've tested Windows Virtual PC, and it's been disappointing pretty much across the board:

1. It doesn't run on our company's Lenovo T61's, which displays some vague error message. Since most of our developers have a T61, this means they can't use it at all. Virtual PC 2007 and VirtualBox run fine, so it isn't clear why WVPC can't. We have some different machines where it does work, so we went on testing on those to see how well it works, in case Microsoft steps up and fixes the T61 problem and starts supporting 64-bit guests. See the remaining points for my experience...
2. The integration features of WVPC suck. Even something simple like dragging a file from the guest to the host or vice versa doesn't work. The reason is that with Integration Mode enabled, you're actually connected to the guest through Remote Desktop Connection. Microsoft has done this to leverage some of the advantages of RDP (as Ben Armstrong explains), but IMHO they should find a different way to do so.
3. Starting/stopping a guest takes forever, and renders my laptop inoperable while doing so. With VPC 2007 hibernating or restoring a hibernated VPC with 1.5 GB of memory takes several tens of seconds, but I can easily use all other applications while doing so. WIth WVPC just restarting a hibernated image tages 3-5 minutes and during that time my other applicaties are pretty much frozen. I can get a cup of coffee (if I'd drink coffee), eat a sandwich, and go to the John, during the wait. I have no clue why this works as bad as it does, because VPC does this perfectly fine.

The only thing that's been a positive experience is the performance once it has started. That said, I can't really tell if VPC 2007 or VirtualBox work better or worse, so I can't even praise WVPC on this point.

So for now it is back to VPC 2007, with our final decision on switching to VirtualBox postponed until we really need 64-bit support.

On a side note, we've been trying to get the open source version of VirtualBox working, and that appears to be quite a challenge. There are no binaries, so you need to compile yourself. Even though we have a recent build, getting all the prerequisits is hardly possible. Some of the needed SDKs are no longer available. Our conclusion up until now: the open source version is  not really viable (at least for Windows), and possible only there for marketing reasons ("see, we do open source").