I held off buying iPad 1 as I wasn't clear about how useful it would be for work. I am a self-employed consultant who makes a lot of compliance visits to law firms. I succumbed and bought iPad 2 a month or so ago. The 'killer apps' for me have been Daylite and iFiles.
Daylite (which I was already using on my MacBook Pro) is a project and matter management system for Macs. It helps you organise clients, projects, appointments etc. It is available for iPad and 'syncs' with my main Macbook Pro. Therefore I can take all of my project information out on the road.
I also use iFiles which provides the iPad with a directory structure for documents - .pdf, .doc, .pages files etc. - all uploaded and read with ease.
.......... use.