Pros: Silent operation, nice status LED
Cons: No chaining of multiple drives
The bottom line: Easy expandable storage. I use mine with Time Machine on Mac OSX. It's quiet and reliable.
I am a Macintosh guy. My iMac has a 320Gb hard drive (about 100Gb free). With Leopard, I am making use of the Time Machine automatic backup. Time Machine works great but it eats up drive space.

I used to use a 250Gb external but it would only hold a few days worth of data plus it was noisy.

The WD MyBook gives me a lot more space and is quiet. It just works... you don't have to think about it. That's what I mean by invisible.

When I plugged the drive in, it showed up and was ready to go. However, since I'm using this on my Mac with Time Machine, I had to reformat the drive with Apple Partition Tables. Since it's such a big drive, the reformat took a while. I should note that I could have used it for regular storage just right out of the box.

Another feature I like is that the drive is intelligent and powers up and down with the system. In other words, when I turn the system off, the drive powers down too. That should help it to last longer.

My only complaint is that if I need to add another drive someday, I'll have to plug that one into another USB port... the MyBook doesn't have an integrated hub so that I can chain several drives together.