It's very difficult for Bing to gain any traction because it does pretty much the exact same thing as Google. The differences aren't compelling enough to switch.
Cuil is really cool, and wants to be different, but it's been hyped up too much for how much it improves. It's a big-budget startup that needs to actually gain traction in order to be successful.
Duck It, from what I can tell is a low-budget engine that brings a slightly different twist to searching (more context). Since it's low budget, it doesn't need to be a home run hit to be successful (like cuil), and it doesn't need to be better than Google at being Google (like Bing). All it needs to do is be good enough at providing context that some small group of people goes to it when the search term's appropriate. If they can get that and build on it, then they can be a niche player. Being a niche player is a success, and at that point they can look into strategies for breaking out of the niche.