If it is your only computing device then yeah, it is tough to do with the iPad alone. The iPad will do a lot but not everything.
I have to strongly disagree with others here who said the iPad is not a workhorse or productivity tool. They either have an unusual use case or don't know all they can do on an iPad.
I have an iMac, MBA and iPad 3. In the office I use the iMac mostly. Good for go to meeting and heavy document creation and editing.
On the road I use the iPad 99% of the time now. My MBA is gathering dust although my daughters use it because the Minecraft version is batter than the mobile version.
What do I do on my iPad? Give presentations, light editing of documents, email, browsing, news reading, forums, some gaming, and video streaming and watching. Frankly, the iPad is better at every one of those tasks for me on the go.
Not only is the iPad lighter and more portable than the MBA, but the user experience is better with an iPad. These is something to touching the screen and directly interacting with your work that can't be translated well with a mouse or touchpad.
It is a different and more satisfying experience for me. I can't wait until my iMac provides something similar and we can mature beyond the complex, clunky computer OS and hardware paradigm. Any laptop is like working in 2003 to me. Bigger, lower battery life, slower, problem prone, bloated and expensive software, limited built in connectivity. Seriously. It's more like 1998 even.