There are 2TB drives out there for less money and if all you need is a huge space, you might consider one of those. The value added in this package is the backup software that is bundled with the drive (actually, delivered right on the drive).
WD Smartware is a backup/restore suite that, once installed, boots with your machine. If the MyBook is attached via your USB port, it will constantly monitor activity and make backups accordingly in the background in near real time without any action on your part. It also maintains multiple generations of files (you choose how many), so you can go back to, say, three versions ago if you need to recover a spreadsheet or other data file.
You can also use it on multiple computers. Backups are stored under folders with the same name as the PC they're created on.
I tested on a Win XP Pro laptop and a Win7 laptop and detected no system slowdowns at all. Every now and then the activity light on the drive would flicker; otherwise the backup operations were not noticeable. The drive itself is completely silent.
INSTALLATION
Strongly consider copying the software included on the drive to a CD before you do anything. I can envision situations where you might want to delete everything off the MyBook and if you do that, you'll no longer have the software to install on another machine (the manual has instructions about where to look for it online).
Plug in the power supply, then connect the MyBook to a USB port on your computer, then turn on your computer if it's not on already. The MyBook will not power up until it is connected to a USB port on a running computer, which is actually handy - if you turn that computer off, the MyBook turns off with it, then comes back up when you power the computer back up.
My Win7 machine saw the drive quickly but complained that the drivers could not be installed. The drive worked fine, though - I was able to create folders, copy stuff to it, and delete stuff from it using Windows Explorer.
My XP machine saw the MyBook and started the Found New Hardware wiz. It asked if it could connect to Windows Update to look for drivers. The drive wasn't showing in Windows Explorer, so I said yes. After a long search for "WD SES Device USB Device," it found and installed what it needed to and the balloon said my new hardware was installed and ready to use.
I then installed the software (all of it) on both machines without incident, by running the WD SmartWare executable included on the drive (blue icon). On installation, I was prompted to perform my first backup, so I did. When I was through with both machines I had two presumably complete backups on the MyBook, one for each machine. For each PC you're supporting with the MyBook, this first backup is your baseline and changes to it will be tracked from that point on.
NOTE: these are not drive images. As nearly as I can tell, WD SmartWare doesn't support creating drive images or recovery boot discs, so you'll need to rely on other tools to create things like that if desired. I didn't see this as a flaw since this isn't sold as a system recovery utility. It will save your rear end if you blow a file or folder, but if your whole machine dies you have lots of other problems to solve before you start worrying about individual files. Of course it's a great location to store drive images, but be aware that the software isn't designed to create them.
I am regularly prompted that a software upgrade is available for the MyBook. Like another reviewer, I found that the installation program complains about additional USB devices being connected no matter whether any are connected or not. The drive works well so I just dismiss prompts to upgrade. Maybe I'll struggle with that later, or maybe not.
OPERATION
It just works. I disconnected the MyBook and then edited files, created new files, and deleted existing files on both my XP and Win7 machines. Then I connected the MyBook to each machine in turn. On both machines, a couple of minutes after connecting the MyBook, the new files appeared in the backup and edited versions (2nd generations) appeared in the backup without my doing anything. Deleted files (I had really deleted them, not sent them to the Recycle Bin) were still there in the backup.
Multiple generations and retention of deleted files is a good thing, but it is also going to cause bloat after time on machines that are heavily used. I can see needing to do housekeeping every so often, getting all my machines to a stable point, cleaning off the MyBook entirely, and then starting over with a fresh backup/baseline of each computer.
OTHER CONSIDERATIONS
This would ideally be connected to a desktop computer and just sit there quietly doing its job. I have three laptops and no longer have a desktop, so for me the advantage of virtually instantaneous file-by-file backup without having to think about it just isn't there because I don't want to be tethered to the MyBook all the time. I will have to remember to connect the MyBook to each laptop periodically to sync the changes on the laptop with the MyBook, but that's not that hard and it's a huge enhancement to the once-a-month (er, maybe) drive image I do now. I need to get off the couch occasionally anyway.
SUMMARY
This is a very well executed software/hardware bundle for those who want to maintain file backups without having to actually do anything - this thing is pure autopilot, and getting back some lost or damaged file that you update infrequently (your will? your online account/password list? your resume?) won't require a complete system restore, which is usually a disaster even if it's actually successful! I'd say it would be of the most benefit to folks who have one computer, or those whose most volatile (data-wise) computer is a desktop that they can attach the MyBook to and just let it quietly do its job, but I am having no problem putting it to use on my primary laptop.