Now, there are a couple of things to keep in mind when running Parallels. First, more is better.
More CPU, more RAM, more hard drive space. I'm running it on a maxed-out MacBook Pro, and I can really tell when Parallels is busy. It eats CPU like kids eat popcorn. Mercilessly. If you have a MacBook Pro, you'll really see it, especially when a VM is first firing up.
RAM is the other resource that Parallels always wants more of. I have my MacBook Pro maxed out at 2GB, and I really wish I could double that or more. To be fair, that's not all Parallels' fault. Rosetta, another necessity on Intel Macs, (at least for the near future) is another memory hog. Try running parts of Microsoft Office, maybe Photoshop and Acrobat, then firing up Parallels. You can get some interesting slowdowns. I'd imagine that the performance is much better on a Mac Pro with say, 4GB of RAM or more, and the dual dual-core Xeons it comes with, but then again, I'd expect it to be.
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Parallels is really quite usable on a MacBook Pro. I'd not want to run multiple VMs at once, at least not on a laptop, but it's not like you have to run Parallels by itself. Just be prepared for it to take a sizable bite out of your system resources.
In fact, Parallels runs really well even allowing for the limitations of a laptop, and the speed hit caused by storing all my VMs on an external drive. (If I want to run Boot Camp and multiple VMs, I kind of have to use an external drive. Laptops don't yet have the option of 250/500/750GB internal drives.)
It's perhaps not as fast as Boot Camp, but then again, Parallels doesn't make you reboot, so it's a more than fair tradeoff. It really is an IT person's best friend. Need to test something in a specific operating system? Does the OS run on modern Intel hardware, or something close to it? If the answer's yes, fire up Parallels and go to town. Need to keep two or three versions of the same OS around? Parallels is your friend.
The ease of testing Parallels has already paid off by making it easier for me to spot potential problems with IE 7, Vista, and Office 2007 tons easier than trying to do it via different partitions on a dedicated system that's not with me all the time.
In short, it's probably the best eighty dollars I've spent since getting my MacBook Pro in August. If you have an Intel Mac, and need to run other OSes, regardless of reason, my advice is: run, don't walk, and get Parallels.