We're doing a book. We purchased a Fuji Instax camera for guests to take pictures of themselves and slide into the guest book we purchased off of Etsy and write us a note. We're not the type to display a canvas or something else with signatures all over it. It would look sloppy in our house, but we're minimalists.
I would love to have a polaroid guest book, but I'm afraid that it just wouldn't work. I would love to have people write messages next to their pictures because that's the kind of thing I love, but I'm afraid no one would do it, or the camera would break, or something like that.
I was just at a wedding this past weekend where they had a polariod guest book and it was amazing. There was a huge blank book and they had really cute washi tape and stickers for you to attach it to the book. They also had a huge bowl filled with film re-fills because the rolls only took 20 pictures. There was a huge line to use it but it was awesome since everyone was helping everyone else take pictures and things like that. My FH and I loved it so much we got home that night and calculated how much it would be for us to do that (we estimated around $250. The camera is about $60 plus all the film and the book) in case we want to add it at the last minute!
We're also doing a photo book with our engagement photos! We aren't likely to hang a big canvas signatures on our wall (or other wall alternatives I've seen) and in all honestly I would be worried about guests signing in a way that left uneven white space or something (I'm weird like that) and a photo book is something we would be much more likely to leaf through in the future than a traditional guest book.