I've been using OmniFocus for the past month or two in its late alpha and beta stages. It's always been very stable and reliable. So far, I've been very happy with the 1.0 release as well.

I've tried Things and took a close look at Midnight Inbox and iGTD2, but I think OmniFocus is the best of the bunch in terms of capability and usability. It allows me to view my projects and actions in the most intuitive way.

Documentation is sparse but adequate and support from the Omni Group is stellar.

The price may seem a bit steep compared to some other GTD apps, but OmniFocus is worth it in my mind. Plus, Omni Group provides some nice discounts. I actually ended up getting OmniFocus for less than most competitors are charging. And OmniFocus is arguably a better app.