The only way I would suggest buying such a thing is if you honestly expect to use more than $150 worth of memory cards on a single vacation (and even then, maybe not.) Remember that the hard drive in such a thing will definitely go bad sooner or later, and it'll be another $50 (bare minimum) to replace it.
Memory cards are more reliable, smaller, and much less fuss. (Just set the full ones aside, instead of taking time to dump them to this device, which then needs to be dumped onto your PC when you return home.)
If you were dead set on a hard drive backup, I'd go for one of the cheaper devices that come without a hard drive (like the $35 one) and then add your own drive, doing that would easily save you over $50 and should work just as well.