I've been using OmniFocus for a while now on my Mac, and made occassional use of it's predecessor, Kinkless GTD. As an implementation of the "Getting Things Done" method, or just as a personal "what am I doing?" manager, OmniFocus 1.0 was pretty good. I, personally, liked it better than the alternatives I tried, but some of that may be due to familiarity with the OmniOutliner heritage. And OmniOutliner remains one of the greatest Mac OS X applications ever written. I believe it's one of the greatest outliners ever written. Sure, it doesn't have Mind Mapping support and it can't run a slideshow (although its data can be exported to OmniGraffle or Keynote). OmniOutliner is a joy because it's fast, beautiful, ridiculously easy to use, while still having a fair amount of power and flexibility. Kinkless GTD was actually a set of AppleScripts that made OmniOutliner act like a specialized GTD application (which OmniFocus now does natively).
But when it came to OmniFocus, I only used it at work, and even there I only used it intermittently. But with the 1.1 "Sneaky-Peek" version of the desktop client (still under development), OmniFocus supports synchronization via MobileMe or any WebDAV server. This not only enables me to share this information between the office, home, and the rarely used laptop, but it enables sharing with the awesome new OmniFocus for iPhone.
Second only to the iTunes Remote, OmniFocus for iPhone is the coolest mobile application I've seen so far on this young platform. OmniFocus for iPhone (or iPod Touch) can synchronize over Wi-Fi, Edge, or 3G. That alone makes it very useful. Cooler still is that it's location aware. "GTD" has a strong concept of contexts. A project may require picking up supplies at an office supply store, assembling at home, and mailing out items at the post office. With OmniFocus, you can add location information to any context. This location information may be based on where you currently stand (using Location Services), on a manually entered address, on an address from a Contact record, or on a business search. Then when looking at the "By Location" screen in OmniFocus, available actions get grouped by their location in relation to where you currently stand. "Grocery Store: 1 Mile; Post Office: 3 Miles..." Very Cool.
This is the first electronic implementation of GTD that actually appears usable. Even if you don't follow GTD religiously (I certainly don't), the projects/contexts combination is an effective way of organizing actions. The location-aware contexts in OmniFocus for iPhone help answer the question "what can I do based on where I am?" When used well, it should make it harder to forget those little items that you needed to do or pick up for some small project.