We've looked at the MyBook before. In fact, we can't seem to stop looking at it. It's a brilliant piece of kit: lovely looking case, spinning blue lights on the front, hidden morse code messages in the ventilation holes - everything you could want from an external drive.
It also tested a tad faster than the other units we tested, although this is likely because our 1TB Hitachi Deskstar test drive sacrifices a little speed for its absurd capacity.
Purchasing a pre-built external drive isn't something to be ashamed of, despite the new simplicity of assembling the construction yourself.
The cheapest we could find a 500GB WD Caviar drive (similar to that contained within the MyBook) was £85. Add to that the price of an enclosure, and the MyBook shines. Ignore the mild lack of flexibility, because this is probably the best eSATA drive out there right now.