So I'm a big fan of David Allen's GTD System. For a long time, I've been looking for a personal information system that could help me get the job done.
I've been extremely impressed with the early versions of Things. I'm starting to get very excited about their iCal sync feature.
If you're looking for an electronic version of David Allen's system, this is the tightest app I've seen for it. I especially like the "Areas" feature, as most apps I've used lack this feature.
Once I have the ical feature, I will be able to sync this info with my PDA. That's really the only problem with Things in its current build. Once I'm able to access this information on the go, I'll feel a lot better about having an offline app for this type of organization.