For example, July 2011 featured concept art for the canceled Herbie the Love Bug attraction!
Members also usually receive an item with each issue. One of the issues last year came with patches showing the classic logos from the original EPCOT Center attractions! I got the logo for The Living Seas. Completed sets of these patches can go for over $100 on eBay!
So is D23 worth it? For me, living 15 minutes away from Disneyland, yes. Most of the D23 member meet ups are held at Disneyland or Walt Disney World. Some events are also held at the Disney Studios in Burbank California (even though I've never been lucky enough to get in to any of those), For a person living outside of California of Florida there are some great gifts as seen in this post, not to mention the fantastic bi-monthly D23 magazine.