Is it a bad idea to use DVD-R's or the equivalent as an external back up system?

It’s probably best to use several different media. CDs and DVDs are good because they’re immune to EMPs, though the jury is still out on how long the substrate will last before it degrades. Ordinary hard drives are pretty reliable these days, especially in a RAID configuration, though an EMP will take them all out at once. Having no moving parts, USB thumb drives are pretty immune to mechanical damage, but again could suffer during an EMP. So again, back up anything irreplaceable to different media.

For me, my stories and manuscripts are irreplaceable. I keep backups on on a USB thumb drive. I also periodically copy them to several CDs (encrypting them in the process), and then keeping one in my car CD changer, and one at my mum’s house for added paranoia.

I’m not sure if you’re worried about storage capacity or safety (both?). The above seems concerned about EMP, and while a possibility not the most likely culprit of data loss. Your’e more concerned with mechanical/physical failure/damage, and theft. Most people wont steal DVD-R’s that have already been used, wouldn’t be very worth it. Thumb-drives and external hard drives, which have higher capacity and are more convenient, are more likely to be stolen. DVD-R’s are also less likely to fail if they get banged around.

Practically, though, I would use an external hard drive. The overall value is much higher, primarily because it’s much more efficient (larger storage, lighter, easier to index, easier to travel with, easier to duplicate, easier to retrieve information from (remember, disks are a dying technology), etc).

I’d use a hard drive instead. They are so cheap these days that there’s no reason not too. I’d also use an external (cloud based) service as well.
I’ve never had much luck with USB thumb drives and wouldn’t use them for anything other than transferring files about. DVDs can also be quite delicate. It doesn’t take that much to damage them and if you can’t read 1 disk then the whole archive could be compromised.

I have a fuckton of data. How I back up.

-Second drive inside computer that gets cloned weekly.
-External USB drive that clones my /users/ folder weekly. Once I clone it goes a few blocks away to keep me safe from something like a fire.
-And I use Crashplan for a online back-up. It works a bit better than Mozy.

I feel pretty safe data wise.

Well, you need a hard drive that’s rated to hold a fuckton (allow 80% additional fucktons to allow for future growth). That is to say, if your current drive has a capacity of 1 ft, you should back up to a drive with a capacity of 1.8 ft.

Personally, I back up to DVD-R myself and make sure than none of my backups are more than 2 years old. I know for a fact that the subtrates last considerably longer than that on name-brand media. My opinion is that those backup discs are more for post-emergency restoration than for truly archival purposes, so I treat them more like a first-aid kit than many people.

Going to Blu-ray would make more sense from the standpoint that they have a considerably higher capacity, but they are also still rather pricey (especially for the media), so I personally won’t be switching for a while.

I find hard drives, whether external or not, are too fragile for my tastes. If I drop a box of properly stored optical media, no problem; the same cannot be said for a hard drive. I would not store them in a spindle pack, sleeves, or normal jewel cases if I really cared about the data; they tend to scratch things. I have a nice hard-case that I could drop down a flight of stairs without losing a bit. I also tend to make two copies just in case.

As for any sort of online and/or cloud solution, I would not use that method to back up any data that I would not be willing to post on my Facebook profile.

a simple external drive is sufficient for me.

Long term yes. External HD seems far more durable.