I run an independent supplement store similar to GNC/Vitamin Shoppe and I constantly use my iPad at work. I use a number of apps regularly that make a use difference.
"Counter Culture" for collecting names and e-mails (We have a very large e-mail data base) and with traditional pad and pen it got impossible to keep up with them all. And you'd be surprised, but at different events that we sponsor, you get home with a large list of names and e-mails and can't read most of them.
Secondly, I use Pages for basic word processing, and ordering/lists for our stores.
Publisher Star, for basic flyers, and banners/signs that we put up in our stores to advertise different sales specials.
PSTouch, now for the same reasons as stated above.
I know it's not iPad dominant but Pocket Scan for iPhone. This app converts photos into .pdfs and works perfectly as a pocket scanner, I've stopped using a fax machine altogether because of this app!
Carpentry app on the iPhone, you'd be amazed at how many times I've had to use this app. It came in really handy yesterday!
EagleEyesHD, great app that syncs all of our security cameras, so I can view them live on the iPad!
At first glade, I didn't think I used very many relevant "work" related apps but when I got down to it, I realized I use quite a bit actually. The iPad for me has gone from being a really fun toy, to a really fun tool that I rely on at work.