People who are mobile (as in floating around an office or travel) whose job tasks are easily attributable to specific applications are very happy with iPads. Ideally, the number of applications they use every day on a traditional PC is less than 5. Examples: Sales, Executives, auditors, policemen, medical professionals, case workers.
We had one group of auditors who literally sent my team a $200 fruit basket in appreciation for their iPads. The card said something like "You changed our lives. Thank you!" Executives spend 95% of their day in mail or approving things... again, perfect use case as long as that workflow doesn't require IE6 :)
If you're a consultant, whose specific tasks may change day to day, a full-stack devops guy using any of 15 tools, or a professional writer churning out copy, than an iPad doesn't work. That's ok... a $500 nailgun doesn't make sense for hanging a picture. We have the luxury of choosing tools these days... in 1999, mobile == 12 lb laptop with cable!