I find OF iPhone is best for capture and execution.

OF iPad is very nice for reviewing.

OF Desktop is required for sharing any information with other people who don't own OF.... or for even those who do own OF.

For example, OF Desktop is the only way I can find to generate a PDF for a Project for people who don't own OF. With OF iPad, even if my co-worker or spouse owns OF, I can't export Projects.

You get pretty locked in. But maybe that's consistent with it being a personal professional tool and not a project level management tool. Although, the first releases of OmniPlan for iPad also had nonexistent sharing support IIRC.