I have a MyBook that I bought a year or so ago to transfer my extensive CD collection to a hard drive after my first collection of 200 cd's got stolen. All the music did not fit on any of our computers, so I bought the MyBook from Costco for about $150 then. It holds the same 500gig storage limit. I'm actually rather happy with it. It leaves all the songs stored there, and Itunes can recall them immediately as if they were just on the computer itself, so I have a huge library on all the computers, but in order to play any of the songs, the hard drive has to be connected. You don't even need to install it. The biggest downside of the MyBook I think is that it has to be plugged into a wall. I bought a 250gig one for my wife, but it's a different manufacturer that we got during the downfall of circuit city for about $50 and it's still working as well. It plugs into the computer, rather than a wall, so it's just 2 USB cables, and can be taken with the laptop and used where there are no plugs.
The MyBook itself does not fail, as far as I've seen. Maybe it's just not full enough, I only have about 80 gigs on there, but it works every time I power the computer on and hasn't erased or corrupted anything yet, but I've heard since it is a hard-drive if you're downloading anything and transferring it to the external drive, it can become virus ridden just like a normal one.
Looking back on it, I think it would have been better to just buy one of those normal hard-drive docks where you take a hard drive and just shove into the dock like it's an IPOD or something. You'd actually be able to store and run programs off it that way.