I've used NetCentrics add-in, eProductivity (<-- loved this! but moved away from Lotus Notes and too complicated to install it on new computer), OmniFocus, a few others, and tried a bunch more. I am now in Evernote using my own setup (I tried TSW, reviewed the DAC version, and tried a few others).
After all this, I must say it's generally not the tool ... it's the implementation, and specifically it's the implementation that really works for you and your technology ecosystem.
So I try new tools every once in a while but my overarching decision factor on whether I invest in setting up a new tool is whether I am more attracted to it than the one I am currently using. If I'm attracted to it, I'm more likely to stick with it and build it as a trusted tool.