A nice touch is the built-in console viewer (see screen image), which displays Windows Server's Server Manager. However, it doesn't work with an OS X Server guest.
I expected storage performance to be an issue, but it turned out to be no issue at all. Windows Server 2008 x64 guests take off like sprinters, and even though Parallels Server for Mac has no mechanism for migrating running processes from one machine to another (you wouldn't expect it for $999), suspending and resuming a virtual machine takes only a few seconds. If you suspend a VM in the middle of a task that uses the real-time clock as a timer, you might not be able to resume cleanly. You also have the option of pausing a VM, which happens instantly, but unlike suspend, pause won't maintain the system's state if the host reboots.
Parallels Server for Mac proved highly reliable. Except for VMs that wouldn't come out of suspend mode cleanly, I experienced no freezes or crashes of either a virtual machine or the host server.
Virtues of virtual storage
Virtual volumes have advantages that Parallels Server for Mac plays to a unique extent. Because virtual volumes can be located anywhere, Parallels Server for Mac can make your client's optical drive operate exactly like a drive attached directly to the server. You can create a new and completely uninitialized virtual machine, slide an OS boot disc into the client machine you're using to manage the remote server, and the server will boot from it. This is similar to a feature inherent in OS X Leopard, but Parallels Management Console makes it much easier to use for remote server installs.
Using an included utility called Parallels Explorer, you can read or alter Parallels Server's virtual volumes while the virtual machine associated with that volume is offline. NTFS, FAT, and EXT2/EXT3 file systems are supported, and as a bonus, you can read and change Microsoft and VMware virtual volume images using the same tool. Parallels Explorer for Mac also lets you mount a Parallels virtual volume image as a local drive so that you can apply patches, add applications using install-by-copy, and relocate system files without destabilizing a running VM.
Similarly, Parallels Server for Mac has cloning and templating features that allow you to create and tune one ideal OS image and either replicate it in full or create a template that applies that VM's configuration to a new install. Cloning a Windows Server OS will trigger Microsoft's licensing tripwire, requiring a unique product key and activation. Microsoft's permissive terms with regard to covering several Windows VMs with one license don't seem to apply when OS X Server is a host.
Parallels Desktop has the ability to mount and run using a bootable physical disk partition, as long as that partition was created by Apple's Boot Camp. Boot Camp is not an OS X Server feature, but Desktop's ability to use a natively bootable volume as virtual machine storage would have opened up some intriguing possibilities.