The main web application of my company is crying out for a nifty set of libraries to make it in some way maintainable and scalable, and one of my colleagues has suggested CS
I had experience with it several years ago. It is a brilliant architecture, but very complex, difficult to understand or change, and it's solving a problem that most of us developing web based applications don't necessarily have. It was developed more for windows based applications and handling multi-level undo, with a heavy emphasis on transactional logic. You will probably hear people say that since web applications are request-response at the page level, it is inappropriate, but with AJAX-style web apps maybe this argument doesn't hold so much water.
It has a very deep object model, and it can take a while to really wrap your brain around it. Of course, a lot can change in a few years. I would be interested to hear other recent opinions.
All things considered, it would not be my first choice of architecture.