I keep trying new tools when they come out, but find myself always snapping back to Notational Velocity. Workflowy reminds me of OmniOutliner, they share a very similar, hierarchical approach. Sadly they also share similar problems; a GUI that's too slow for my fingers and too complex for my little brain. The GUI-complexity stems from the enforced hierarchy. Indenting stuff seems nice at first, and makes it look tidy…
Sometimes the tool is irrelevant. I have one friend who used a directory structure and text editor (pretty simple) to create an enormous cluster of tasks he'd never get to. Many thousands, like the Windows registry after a couple years of random software installs. His issue was just needing a mechanism to periodically clean it out, so a change in approach rather than a different tool.