The other key factor in replacing a computer with an iPad is a useable keyboard. It's true that the iPad has an onscreen keyboard, and I've heard there are some people who are adept at using it to write email, send text messages and create word processing documents. I'm not one of them, and apparently I'm not alone in my sentiments. If you're going to try using an iPad instead of a computer, you need a real keyboard. There are now several BlueTooth-based keyboards to choose from, many of which come in an integrated case that also holds the iPad, turning it into a smaller, lighter version of a laptop.
Initially, I bought my iPad as a reading device, and for meetings. Now I use it more than my desktop machine -- and as a complement to my desktop machine.
For everyday work, I use Apple's standard Bluetooth keyboard, with the BookArc stand. The iPad works brilliantly as a second screen.
My tasks for the day are set out on OmniOutliner for iPad, as my outlines for writing projects. I can type away on my Mac, while glancing at the outlines.
Do you use your iPad for your writing?