My requirements are a bit less intense than others since I am not a professional. Were I to lose data, it would be very sad but it would not be an adverse effect on my income. That said, I've been using tow 1 TB drives in a mirror RAID. I've had 1 drive go down and simply replaced it. Since it is possible that both would go or that there would be another type of catastrophic failure, I supplement this with a periodic external backup where I rotate 2 external drives.
Now, the reality. I don't do the external backup often enough. Someday, it could burn me. In fairness, I don't generate too many new files except after a trip. But still, this could be a problem.
Human nature being what it is, I am therefore trying out Crashplan. The initial upload is gonna take days. Then, I'll see what happens when I add some files.