Now that Apple's Mac range is based on the Intel processor range, the architecture challenges for emulating a PC are considerably less significant than back in the PowerPC days. Fusion 4.1 is VMware's current offering in this marketplace.
Installation's a simple process, and because you're running on a Mac there are no issues with needing drivers for wacky hardware. My test machine was a MacBook Pro with a 2.53GHz Intel Core i5 processor and 4GB RAM.
Once the package is installed, you can run up your guest operating systems. I started with the 64-bit flavour of Debian Linux 6, which we installed via Debian's network installer; so you download and boot from a 180MB ISO CD image and then it installs its 1,000 or so sub-packages over the Internet.
On my 2Mbit/s ADSL I simply left it to install overnight, and in the morning I was ready to rock. As expected, the resource requirement of the Linux install was modest and I didn't really notice any impact on the speed of the Mac's native operations when running Debian on a VM.
The next obvious step was to try Windows, in my case Windows 7. As with any Windows install this isn't a ten minute job, but it wasn't a big deal as I could happily switch between native Mac operation and the Windows installer in the VM (for example when the latter was demanding input). I set aside 25% of the system's resources for the Windows VM, and apart from the occasional pause for breath (primarily when accessing the hard disk, it seemed) native operation wasn't really impacted.
When using a VM, the cursor is "trapped" within the window representing the VM you're currently working with. To flip back to the native Mac GUI, you just hit the Command-Ctrl key pair. Networking is dealt with for you; I didn't bother with any custom settings, so VMware provided a DHCP facility in the private 172.16.x.x address space, NATing it into my 192.168.1.x/24 local range and thence on to t'Interweb.
All fine so far, then – but would I really want to run a PC in a VM on my Mac? Particularly as right next to the Mac is my meaty Athlon-equipped HP laptop.
Yes, absolutely, I certainly would, since if you're a relatively mobile user like me it means you can take many systems with you in a single box. You do, of course, have to be mindful of your backup needs in this context, but you should be dealing with that anyway.
So are there any downsides at all?
Yes, of course, and they're all related to resources. A single hardware platform means a single set of hardware, and while some are shareable (e.g. the netwwork connection), if you want to run multiple VMs at once you need to consider single access to resources such as CD/DVD drives. And of course you'll always have the trade-off, again particularly when running multiple VMs concurrently, of sharing resources between VMs and the native OS.
I tend only to run one or two at once, though, so this isn't a great issue – and the resource allocation functionality is a doddle to use anyway, so once you've spent an hour or so fiddling with the settings, you're likely to have it about right.
Vmware Fusion is, then, a highly attractive choice for running multiple guest OSs on a single hardware platform.