Personally and in my conversation with folks, I find that as a productivity device, iPad users fall into 2 buckets.
Bucket 1: Creators: developers, designers, writers and such - folks who need to create documents and other work product. Broadly speaking, in terms of available apps, and interconnectivity with apps (eg leaving Photoshop to go research something in a browser and coming back, copy paste workflows etc) the iPad is simply not there yet. For stand alone workflows, it goes some of the way there (Eg if all you're doing is writing, you can probably get away with it), but if your role involves using several tools, the tools simply arent there yet.
Bucket 2: Decision makers. I know a few successful CEOs who only work on an iPad. For them, most of their workflow is digesting data and decision making - they arent writing code or making slide decks. They just process emails, do product review and look at data and conclusions. For them, the iPad is perfect because all those workflows are easily completed on an iPad.
I think the iPad will get there eventually - the type of creation that is done will shift somewhat, and the tools will get better, but for now, even the most dedicated folks have a hard time using an iPad exclusively.