I’ve tried Smart Ideas (for which we have a site license), CmapTools (open source) and reviewed Inspiration. While all are good brainstorming / concept mapping tools in their own right, none match the simplicity, effectiveness and pure visual beauty of Bubbl.us. It’s completely web-based, cross-browser and has a very slick interface. It’s great as a teaching tool when you do not want the hassle of installing a fat brainstorming tool (or worse getting your students to install it) in order to do the occasional brainstorming session. All this value and I haven’t even touched on its sharing and collaborative features. Give it a try for yourself…
I agree, I have had the pleasure of using Bubbl.us on a few projects and it is a really wonderful tool. For the most part it allows for everything I need to do, and as you said keeps things simple and clean which is important. You might also try out MindMeister if you haven’t already, its another great mind mapping tool that I use frequently.