For the past few weeks, I've been working with Parallels Desktop for Mac OS X on my MacBook Pro, and I have to say, I'm impressed.
First, note that I said "working with" instead of my usual verb for testing software, "playing with." That's a deliberate choice, because I've been getting work done with Parallels from the start. Currently, I have two Virtual Machines for Windows, one running XP SP2, and the other running Vista RC 2. I'm in the process of creating a Linux VM running Ubuntu as I write this.
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While I did set up my MacBook for Boot Camp, and installed Vista on that partition as well, I've not had to reboot into that partition in weeks. Parallels has been useful enough to me that I haven't felt a reason to reboot just to test out Vista functionality, or things like IE 7 under Windows XP.
It's been as convenient as I'd hoped it would be, and with a couple of exceptions (like drag and drop of files to and from the VM and some other minor things), it's as nice to use as Virtual PC Mac ever was, and obviously much faster.
Installation is simple. Run the installer, reboot, and go. Creating VMs is pretty brainless. Select the type of VM you're creating, pick where you want the VM to live, choose some basic settings for max VM file size, max RAM usage, and then Parallels is ready to go.
If you want, you can set other parameters, like the source for a floppy drive, which drive or image to use for the CD/DVD drive, your shared folders, (shared folders don't work with Linux VMs, unfortunately) and other settings. Give the VM a name, and the setup is done.
Yes, there are a lot of tweaks you can do for the VMs. I haven't bothered with them, and I've yet to feel a need to bother with them. The default settings are good enough for my needs, and I certainly can find no fault with the simplicity of the setup.
From there, you just install the OS into the image as if it were a standard Intel machine, and you're off and running. One tip: Don't install from actual CD/DVD media. Make an Apple Disk Image, or an ISO image of the install media, and install from that. The speed difference on the install is like night and day, and it keeps your optical drive free. If you end up having to reinstall an OS into a VM, installing from a disk image will keep your sanity about you much better.