It does seem pretty common for external backup hard drives to fail, but although Kees_B says "be prepared it will fail" I think the point is that you cannot rely on any device to be 100% immune to failing. Personally I think how you treat devices plays into it in large part - a small external hard drive (particularly portable ones that get carried around a lot) are going to be subject to abuse more than a tower that just sits there. I would equate this to a degree with laptops - I have heard a lot bad things about Acer laptops, but I bought one anyway because the price was right for the features I needed. From a reliability standpoint it has worked superbly and I put this down in part to the fact that I treat it well. I don't bang it around, I use compressed air to blow out the vents once in a while, etc. It hasn't let me down (while I have friends with the 'big name' laptops that haven't fared so well).
Anyway, if you purchase a backup drive and treat it well (don't knock it off your desk, don't put it in a bag and sling it around, etc.) and you should be fine. That said....... expect it to fail and don't rely on it 100% for storing anything you want to keep. Always have another copy. That way you won't be disappointed. Buy an extended warranty as well - usually a reasonable investment.