It seems like the market for html/css tools is so vast, it doesn't seem hard to build yet another tool that can end up being profitable. I've been working on a grid-based rapid html prototyping tool http://pageblox.com and I was looking to incorporate live css editing similar to this into it. Of course Fog Creek put a twist on it I never would have considered (editing/modifying css of an already live site). I'm very curious to see how (and learn from) how this will evolve as I assume this is their feedback/iteration phase.