I use NoteTaker and I love it. I too used Omni Outliner but found it quite limiting. NoteTaker is very flexible. Plus, you can set up clipping services so that you can save email tidbits into your notebook, and searching services that will go into your favorite search engine (I haven’t tried it yet). There has been an excellent series of articles providing in-depth review of outlining software from http://www.atpm.com/.