One thing I'd recommend doing is running sysprep on the base image to make sure the SIDs are unique for your virtual pcs.
Thanks for the tip Phil.
TIP: Once you are happy with the base disk, you can make it read-only so that you don't change it by mistake.
THE BAD:
The one thing I've discovered using them that irritates me is that the various security updates are so fast and furious that the original base disk becomes alomst useless over time. So sometimes, if I just want to set up a quickie virutual box and don't need Internet access, I'll just set it up from a diff. disk, but set the network adaptor as "Not Connected." That way, I can get something quick set up w/o having to worry too much about security.
I've been using both standard virtual disks and differencing ones for quite some time now, and found that differencing disks tend to end up using more disk space. There seems to be a fundemental difference between the two types of disks:
When you write a specific block to a standard virtual disk for the first time, it is being allocated (just-in-time), and then reused each time new data is written to it.
The diff disk is basically a log of all write operations. So if a block is being re-written, it consumes more space with each write.
So diff disks are great for undo disks, where you want to change stuff and then commit or rollback, causing the diff log to be cleared. But they are not so great for continuous operation.
For example, I was using both a diff disk and a standard virtual one for about 5 months, using both for Windows 2003 + SQL + doing some development on Visual Studio. They both spawned from the same root 5GB virtual disk. The diff disk reached 13.6GB when I quit using it. The standard disk used only 6.3GB.