I've been using OmniFocus since pretty early on in the private beta. It started out pretty good and has gotten very, very good. For years, I have said that nothing is as effective as index cards. OmniFocus replaced my index cards a few months ago.
There are things that they can do that it cannot (for example, go in my pocket while I walk around), but its ability to organize tasks along many axes very simply and very effectively has been great. I sure wouldn't mind some improved printing options, though.
OmniFocus is OmniGroup's take on the Getting Things Done method for organization. It sprang from the Kinkless GTD system that an eager user built on top of OmniOutliner, but is much, much more powerful. I don't want to rehash all of GTD here, so I will summarize:
So, you don't work on the "remodel the third floor" project all at once. You work on all your "At the Hardware Store" tasks, then the "At Computer" tasks, then "Working around the House" tasks, which means that you may work on many different projects at once, but all in the same way. It has worked well, for me.
OmniFocus makes it easy to enter new tasks, providing quite a few ways to do so and eliminating most excuses to avoid keeping track of things to do. It also makes it easy to flip between planning and doing, which is very importing when, while working, you realize that there's more to do than you thought.
Its view-filtering facilities are quite good, and I think they'll get a good bit better before release. It lets you save the views you use the most and call them back when you want them. It applies simple styling to tasks, letting you see at a glance what you can (and should) work on next.
There are still some quirks to work out -- it's beta software, at best -- but even if it doesn't get any better, it's very good. It just went from private to public beta, and they've announced some more details. It will be published in January, and will sell for $80. That, I think, is a pretty damn high price. I don't think I would possibly pay that much for OmniFocus, even liking it as much as I do. But! You can buy a license now, before it gets a 1.0 release, for $40. This is a good price. I've been telling friends and family to check it out, because it's worth getting at $40.