I own an Intel-based Mac Mini and a BlackBook. I run Boot Camp on the Mini and Parallels and Fusion on the Blackbook.
In short, I don't see how anyone other than a casual user can only live with one OS. The simple truth is there are good and bad things about both systems. As an example, if you work on a corporate network, XP or Vista is a far better network client than OS X. On the other hand, if you're handling media, the Mac excels with iLife. If you're in an Exchange shop, Entourage just doesn't measure up to Outlook as an e-mail client. And so on and so on.
One surprise I've found is that running Windows XP or Vista on a Mac is a far better experience under Boot Camp than it is on a Dell, HP, etc. I'm not an engineer, so I can't give you the reasons for it, but my guess is that the drivers are created by the same people who create them on the Mac side, so they just know how to make them work without a hitch. Combined with good Apple engineering, the Windows experience using Boot Camp on a Mini or an iMac is really something to write home about! Someone asked Walt Mossberg the other day whether they should buy a Mac to run Windows even if they never really intended to run OS X. My answer to that question would be an enthusiastic yes, it is a superior experience. I'm not a gamer, so I can't speak to that specialized segment of Windows users who require special display cards and huge amounts of horsepower, but for just about everyone else, you won't find a better computer. Just add a PC-mapped keyboard and mouse and you're ready to go. There is, of course, the added bonus that if you should ever decide to see what a Mac can offer, you won't need to do anything other than reboot!
As for Fusion and Parallels, I think they are both top-notch products. I have settled on Parallels as my engine for running virtual machines, but either one will give you an outstanding experience. If you're thinking of going this direction to run a Windows machine, make sure you install the maximum amount of RAM (memory) and the largest hard drive you can afford to optimize your experience.
Two notes in closing. There is no need to purchase RAM from Apple who charges an outrageous (an unjustified) premium for memory modules that are generic. Buy your memory for an iMac, MacBook or MacBook Pro from a third party provider like Crucial. You'll save a bundle and memory is easy to install in any of the above machines. (You should avoid doing this with the Mini as it is difficult to service unless you're really experienced with this particular computer. It takes a special knack to remove the case and inside everything is packed very, very close. It's not an easy computer to work on.)
You should also purchase hard drive upgrades from a third party vendor, as once again, Apple charges full list price for all of their drives. These drives are not Apple manufactured products. As an example, my BlackBook had a Toshiba 80 gb drive. I recently upgraded my BlackBook with an Hitachi 200 gb 7200 RPM drive. Using a program like SuperDuper (which is free) to create a clone of the existing drive it was amazingly simple and the new generation of 7200 RPM drives won't drain your battery any more than their slower 5400 RPM cousins. This was an inexpensive upgrade to my BlackBook and has more than doubled my hard drive space, plus made it perceptibly faster. Instructions for this can be found by Googling on "Mac hard drive upgrade" or similar words.