I have to say it is quite nice having integrated Windows XP Mode from the Start menu. Bit slow when you load the first application but after that it is reasonable to use.
You can also create multiple XP VMs based on the same base install - so no activation issues (and presumably add other Windows such as Vista and 7 though I haven't tried that yet) - and have installed apps appear in the start menu too. Neat solution.
VMWare is less convenient because you have to start a machine manually to activate an application (and not free). Having said that Unity mode makes it pretty much the same as the MS solution once you have a VM open. I have to say that VMWare's virtual machine is much quicker in use than the MS Version and there is more flexibility when it comes to clients.
I didn't find out till after I committed to buying a new setup with Hyper-V support
At least you can use the VMWare ESX client now so that you can virtualise your whole system if you want