Probably 75-80% of my hits/kudos come from guests, so I love the option being available for them. I'm pleased they have a way to let me know they read and enjoyed, comments or no. I know I've often wanted to leave a "liked this!" note on a fic from a site where I don't have an account, and I'm not going through the rigamarole of signing up just for that ONE fic. I also think the kudos system helps to get around the "oh god, if I leave a comment, is this going to start a conversation, I don't want to CHAT, I just want to say 'nice fic' a;lsakjdf" feeling that comes up a lot on LJ-type sites. Definitely pro-kudos, me.