I'd dare to guess that most people who say you can't use an iPad as your sole computer have never tried it.
I went all iPad about 4 months ago and it's been great. One thing I do ALOT is writing and with my bluetooth keyboard and dropbox, I'm set.
I don't do much else but surf the web, watch video's, and e-mail. In fact, for my purposes, an iPad is much superior to a laptop. I love being able to read books and watch video's on my Ipad as opposed to a laptop. To show you how much of a non-issue processing power is I'm even using the ORIGINAL iPad.