I am using an iPad 2 and an iPhone 4S for my work. The iPhone is private but I have to admit, that it is so easy to combine these two devices that it is hard for me not to use it for work as well.
I have a regular desk job with a laptop (MacBook Pro) but I do have to visit customers every now and then. And I can say that I can do so much on the iPad that I leave my laptop at work more often.
Mail
Of course Mail is the standard app I use every day. All my mailboxes are IMAP so I have all my mails at hand when I need them. The new multiple signatures in iOS 6 are amazing and a feature I have been waiting for ever since I had an iOS device.
I guess there is nothing more to say about it
Evernote
I was a bit skeptical about Evernote when I first saw it. My mom showed it to me on her iPad (she became an iPad-only user ever since she has it
) and I couldn't quite figure out if and how it could help me in my daily routines. But I can tell you: I don't know how to live without it anymore! I am using it on my work laptop and iPhone as well. Not having to email me stuff anymore (and I do remember those times) is so amazing. Whatever it is text, pictures, documents all on all devices and at my disposal. As I said before, I can leave my laptop at work and still have the important stuff with me.
GoodReader
I first started with the basic Dropbox app but I am using GoodReader most of the time now. GoodReader reads all my documents and with the Dropbox-synch I can download them all and use even large ones while not having a wireless network at hand.
NoteTaker HD/Penultimate
Trying to reduce the amount of paper I need I am using both NoteTaker and Penultimate. NoteTaker has the way better organization system with folders and the impression of have a couple of pages on one topic hold together by a paperclip. It even has more pen styles and types (like the marker pen). But I still have more the feeling of writing on paper when I am using Penultimate. This is why I still use both and depending on the type of notes I have to take choose the more suitable one. I am really looking forward what Evernote will do with Penultimate, now that they have bought it.
iWork
I am not a big fan of Pages on the Mac but the iOS version is amazing. I used DocsToGo before and was not very happy it never felt good. Until I started using Pages. The import of Word documents is exactly to my needs and it does everything in a smooth way. The iCloud support is of course a super feature.
Keynote helps me to start a presentation on the go and style it later on my laptop. I even have the iPhone control app.
I have Numbers, too, although I don't use it that often.
Cubetto
Being a programer at a consulting firm I use our own app quite regularly in meetings. It is a business process modeling app with the ability to model different modeling languages as BPMN 2.0, Event-driven Process Chains and even mindmaps. The touch interface makes it much more suitable for meetings than any other method I tried.
The iPad makes work much more easy. I don't want to miss it anymore
Johannes