Merlin Project
I recommended OmniPlan a few days ago. I take it back - use Merlin Project.
This is an addendum to my piece on How To Juggle Multiple Projects. In that post, I talked about OmniPlan as the bedrock of my organisational "system". As Sod's Law would have it, OmniPlan duly crashed out on me a couple of days ago - an App Store update locked me out of the Pro features, while frustratingly acknowledging that yes, I had bought the upgrade and yes, I was entitled to use the features, it still wouldn't let me in. OmniPlan responded pretty quickly to an email and have put in place a temporary fix while they patch the update.
But for the day or so while it wasn't working, I went searching for an alternative. And I found one, and it's a lot better and cheaper (while still not being actually cheap).
Merlin Project may not have the fun animated tutorial videos of its competitor, but it beats it in every other category. Merlin is much more intuitive than OmniPlan, easier to input projects into, more flexible and it also adds Kanban Boards and Mind Maps into the mix. You can brainstorm a plan in several different ways and then quickly turn it into a schedule.
If you were toying with trying out this system and you haven't committed to an app yet, check out Merlin over OmniPlan.