While I'm not a fan of GUIDs as clustered keys, I've seen it in a production system and it wasn't the end of the world. We did have to deal with some excessive fragmentation issues and the obvious problems of fewer rows stored per page were there. Performance was reduced. It's absolutely not a choice I would make, but if you're already neck deep into that style of design, I wouldn't recommend a complete rewrite and redesign, but you should sure as heck go into it with your eyes really wide open so you're not surprised by some of the issues that are bound to come up.