I have the Canon s600, and it has the individual ink tanks (Smart Tank, Think Tank ?). I am unsure just how much money it saves me because, in my experience, the tanks run out within minutes of each other anyway. I tend to print out only pictures of the kids, so I believe that they use each ink at a similar rate. If you print out certain graphic designs that are predominently one or two colors, I can definately see how this could save you a lot of money. Even if it does not save a lot, I would have a real problem with throwing out a three-color ink cartridge that had 25% blue left in it.
As an aside, the s600 is a great printer that you could probably get for less than $100 (it has been replaced by the s630). On the right paper, you cannot even tell that it isn't a real photo.