At this point I use my iPad for most common tasks, but need the power of my laptop for virtualization tasks as well as creation of content, web editing, and coding. And I didn't expect my iPad to take those roles (though I would love to come up with a way to leverage it for even more).
One thing I do find, though, is for me a 3G capability is clearly becoming more and more of a must have. I fully expect to grab one in the next few days, migrate myself to it, and either sell or "hand-me-down" my WiFi iPad to someone. There are many times I am out and about and reach for my iPad, and have to find a wifi connection. With ubiquitous connectivity comes even greater usage cases.