I have many gigs of digital photos and I have also more-or-less moved away from optical media for backup and switched to HDD. As the original poster mentioned, most of the "information" you find on the net about archival longevity of optical media is personal anecdotes or pet theories, and good hard data on archival longevity of CD-R or DVD+-R is hard to find. My own personal experience is that name brand discs do have fewer problems than cheap "house brands", but it's hard to quantify or say much beyond that.
Backing up to hard drives has a number of advantages:
1. It's a heck of a lot easier - in most cases of personal data backup, a few 1TB HDDs will hold all the data you need to back up, so there's no need to manage boxes of 100's of discs. I usually back up the same data onto two HDDs, and store one of them in a firesafe. If you're really worried, you can store one of them offsite.
2. Since no media will last forever, you will *always* need to roll your data over to new media every so many years. With HDDs, its *much* easier to roll your data over to new media every 5 or 6 years. Think of transferring two or three HDD's to a new HDD (by the time you roll over the data, the new HDD will probably hold all the data from those two or three older HDDs), compared to re-organizing and re-burning hundreds (or more) of CDs or DVDs.
The bottom line is that if a few HDDs don't hold enough data for your needs, then backing up to optical media will be totally out of the question anyway, and you will probably need to use tape.