I recently obtained the card in addition to renewing my passport book.
I felt that the online renewal price of $30 was worth it to have a back up Federal ID. When I travel I plan to keep the passport book in the hotel safe and to keep the card on my person as it is proof of citizenship.
I used to keep copies on me and the book in the hotel but then I realized it would just lead an authority/poser to my hotel... (I travelled in Colombia many years ago when it was very likely one would be randomly asked for ID).
I am headed to Cuba now so I think a second form of ID will be a good idea.
For those who frequently take trains or drive to Mexico or Canada, this card is also a good option. Like the traveler above, I also keep scanned copies of my passport readily accessible online, and before I went to Colombia, I also gave a couple of color copies to relatives (there were no cards back then) just so that if I lost the passport I would have multiple resources to access it's facsimile.
I feel the card is just a better and more reliable back up than copies, though I do intend to keep copies as well.
I can waste $30 just walking down the street in Manhattan, so for me, I think this is money well spent, the worst the card can do is nothing, but if needed, it will be worth every penny.