I bought one of these to support an external 750GB RAID set up (3 drives bodged together, in effect) that stores all of our personal data - pictures, music, documents, etc. and quickly changed my mind... in a good way!
The drive is small and neat, and automatically appears on the Mac OSX desktop with a getting started guide - a nice touch for non-technical users. I formatted the drive (choosing to zero out the data - to make sure the whole disk was OK - which took 10 hours! A normal format is a matter of minutes, but doesn't check the whole drive) and backed up our data to it.
The copy process was relatively quick (pretty typical USB2.0 to USB2.0 transfer speeds; some 500GB in about 6 hours) - and was very quiet, particularly when compared to the existing drives.
The base has a blue light on each side which flashes when the drive is accessed - not too obtrusive, but it would be nice to be able to turn it down or off.
In any case, after a day or so of use, I decided to get a second drive - to replace the old setup. I've got the two drives side-by-side, set up to "mirror" each other (using the Mac OS RAID functionality in Disk Utility); this isn't a backup solution as such - but it provides protection if (WHEN!) a drive fails, as the data is duplicated. If one drive breaks down, then I can just replace it and carry on without any loss.
On top of this, the drives come with a 3 year warranty from Hitachi, so between that, the data mirroring, and the Time Machine automatic backup process, I'm pretty well covered. Unless the building burns down, in which case I'd need offsite storage too. Hmm - maybe I should buy another to leave in a drawer at work?
All in all, a great purchase so far - time will tell how reliable the disk is, and how good Hitachi's warranty process is - and at this price, an absolute steal; it seems that every time I buy a new hard disk, they've doubled in capacity and halved in price... not that I'm complaining :)