Omnigraffle really fails for me on a bunch of levels.
Firstly the interface is pretty unintuitive, I really don't want to read through the documentation to figure out when I should use a project, vs sequential actions, parallel actions or folders.
The 'note' pane in the inspector is pretty basic, and offers no formatting options (or at least no obvious ones) to really describe the task.
It'd be great if OmniFocus integrated with Calendar and Reminders in a meaningful way, and could do location based alerts. Like i'm in San Francisco with customer X working on projects A, B, C, or i'm entering into meeting Y at 15:00 GMT-8, which tasks to I need to complete before then.
A large part of that could be solved if contexts autoswitched based on location, but they're just not smart enough currently.
There's also no options for sharing tasks with other people in a project. I get that it's meant to be more of a personal project management tool, than an actual project management tool, but there's times when this would be a really useful feature.
It's really best to consider this as a personal time management tool, rather than a proper project management tool. In that context it works well even if it may feel a bit unfinished when using some features.