I'm using Toodledo on the web and syncing it with Appigo Todo on my iPhone and am very happy with it. The system works flawlessly for me.
One of the nice features of Toodledo is that you can email tasks to it. While Outlook allows you to drag-&-drop an email to a task, it's just as easy to get the same result by forwarding an email from Outlook to Toodledo.
Toodledo is very flexible so you can configure it to work in many ways. What I've found works for me is to create folders (lists) based on task status, e.g. Next, Project, Waiting-for and Someday. This structure works nicely with Appigo Todo. Note that toodledo has its own iPhone app too but I'm not a big fan of the interface, although others are.
I posted screenshots of some of my Toodledo screens in another forum post that you might find helpful: