Free is always fun, especially a tangible item like this. The giveaway encouraged me to go to Moo (which I probably would not have done), spend 20 minutes cropping my pics and filling in my info, and develop a user-opinion in the meantime.
My opinion? It was mostly a neat site. Some issues I liked:
Ability to pull in your sets
If it were just my thousands of photos, I'd not have taken the time. However, because they pull in your tags AND your sets, choosing was super easy.
Working Back Button
I can't tell you how much more convenient the back button is when a thumbnail is all you've got to go on... So many of these Web 2.0-ish sites can't allow back because of postdata issues. Glad they kept it.
What could have been better: If the images grew when you hovered, that would have saved me some time.
What if I wanted to add an image that I had just taken? I'd have had to go to Flickr and then back to Moo. That's too many steps...
For the most part, the site experience was great and I look forward to seeing how my picture cards turn out. Go grab your set (if you are a Flickr Pro user) at Moo.com/flickr before they run out. They're only giving away 10K sets.