I recently tried it - just lap size, and maybe you can learn from my mistake: at one point, early on the second run my bobbin ran out and I sewed along a good 3 yards before I noticed. This must be some kind of record! Since the seams are so long, and I was letting it fall off the back of the table beyond my line of vision, it was too easy to overlook. Other than that, it was fun. I have it sandwiched and when I get it done I'll post a picture. I don't know if I'd have the patience to do a bigger one.
I like the spontaneity of this idea, but I also like that you do have a little choice about which two sides you're going to sew together each time.