All of the Omni iPad apps are intended to be useful standalone products, and I successfully use them in that fashion. However, in all cases, the combination of iOS app + Mac app is significantly more flexible or powerful than iOS app alone, and if you are unable or unwilling to go there, you should make sure that you'll be satisfied with the subset provided by the iOS app alone. New functionality is added on a regular basis, but if truly you need feature XYZ, it may not be prudent to purchase until it is announced that XYZ is now shipping. For various reasons, Omni prefers not to make forward-looking statements about pricing, features, schedules, and the like, so if XYZ isn't available now, it is difficult to make firm plans which rely on it.
On to some specifics about OmniPlan for iPad. Yes, you can develop project flows with Gantt charts on the iPad version. It's a bit different in feel from the Mac version in that you don't have a separate outline view of tasks, milestones, etc. but rather just the Gantt chart. There were some requests in the beta for some means to see just an outline view, perhaps as an export, but if there are plans to provide such a view, I've seen no evidence yet in the beta versions I've been testing.
Also missing besides the outline view is the ability to paste a list of tasks from another application to serve as the initial draft of a plan. What you'll get if you try it is a single task with multiple lines (and you'll only be able to see the extra lines when the task is selected). This is going to be more of an inconvenience if you typically start with some other document; if you start by free associating a list of tasks and milestones, this isn't much of a hindrance at all.
One area of significant concern is the ability to share project plans with others. OmniPlan for iPad is not yet a fully-fledged tool in isolation for this. Whether this is a deal-breaker or a total non-issue is something you'll have to decide. Arguably, some of the issues in this regard aren't any different even if you do have the Mac application —*if you are the only one who has the Mac application, your collaborators will still need your assistance to update the plan, for example. As of this writing, OmniPlan for iPad 1.1 is in review at Apple, and assuming Apple approves it, it gives you PDF export, task and resource reports in HTML, and printing. You get the same Task report as from OmniPlan for Mac, but for unknown reasons, the Resource report is pretty much useless — it doesn't break down the tasks, unlike the Mac version. I'm suspicious that this may just be a bug which was overlooked, and have filed a bug report.
Finally, there are some tools for leveling, rescheduling, and filtering which are not as powerful on the iPad as they are on the Mac. It is my hope and expectation that some of this is simply a matter of "we need to start somewhere" because in all fairness, the original Mac version also lacked a non-trivial amount of these features. Some of it is probably due to the differing styles of user interaction between the two apps. Some of it may be features that they feel unnecessarily complicate or clutter the interface. The items that come to mind as most missed from my point of view are the reschedule button (moves all the incomplete stuff from the past to the future), the ability to combine terms for filtering, and the ability to specify a task to use as a template for creating new tasks.
All in all, I think it is a handy app to have, and I'm glad it is finally here. I recommend that you time your purchase so that you can use it intensively in the first 30 days and make sure it meets your needs while you are still covered by the money back guarantee.