I think it's worth every penny. OmniFocus is by far my most used iPhone app. Reasons include:
- Feature rich (more capable than competitors)
- Very reliable/robust (easily handles my ~400 projects and ~2500 actions)
- Frequent updates (app is clearly receiving lot of attention from the developer)
- Syncs over the Internet (not just LAN syncing like most competitors)
- Beautiful integration with Mac version
- While not perfect, quality is very high
As far as I'm concerned, OmniFocus paid for itself a long time ago.
-Dennis