Backing up data, specifically photos in our case, is an on-going chore. It's even more complicated depending on what you choose to back up to, and how you do your back-ups. Indeed, what you back up to can be half the head-ache these days with all the information out there about what's more reliable and how long each media will last.
I use a combination of hard drives and DVD's. I download to my working drive, and then use a great little program called Allways Sync to make sure everything from my working drive gets replicated on a duplicate back-up drive. If it's a paid job, or something else that's of high importance, I also then burn a set of DVD's.
Perhaps a little bit of over-kill, but it lets me sleep at night, and that is the key. What will let you sleep at night? Or, in other words, what are you comfortable with? How much or how little redundancy in your back-up plan is enough to give you peace of mind?
I have photographer friends who have eliminated DVD's from their back-up plans altogether. I also have photographer friends who are entirely DVD based in their back-ups.
The facts seem to indicate that DVD's do not have a great life expectancy and are certainly not an archival storage media. This, of course, depends on a few factors such as the brand of DVD, whether you get a good burn (i.e. is your burner still doing it's job 100%), and how you store the DVD's you've burned. All of these factor into how long you can expect your burnt DVD's to be readable.
Of course, some make the argument that with the speed at which technology is moving, the likelihood that DVD's will at least last until the next new media format (i.e. Blu-Ray)is good. I'd have to mostly agree with that, bearing in mind that I hedge my bets and use hard drives too. :) In any event, it's pretty well agreed upon that DVD's are not what you want to be counting on for long-term archival storage.
So, a mix of both hard drives and DVD's, just hard drives, or maybe just DVD's with several copies and a plan to up-date them to the next new media... take your pick. What will let you sleep at night?