I have one of these and use it all the time to quickly back up during the day since I had a CF card failure that cost me a days work.
The issue with them is not the HD capacity its the battery capacity. The battery only holds about enough energy for about 120Gb of back up. I have upgraded the the Li Po battery in mine and intend to swap the 120Gb drive for an SSD to make it more robust and give it a bit more battery life. Having said all that up till now I have never shot more than about 60Gb in one day and would never keep more than a days work on it. You can run it from a car USB charger or USB charging it from a USB takes a long time though.
All the rest of it is good, it seams robust, the interface is intuitive and you can quickly do incremental back ups with a couple of button presses.