My iPad has gone with me on every overnight trip (and many other trips) since I got it. I import my routes, save my maps, and use the GPS (which isn't on the wifi-only models) to quickly pinpoint my location and get me back on track (or keep me there). Gone are the days where I had to find a labeled cross road, and then comb the map to figure out where I was, hoping that the crossroad signs had the same names on them as the map.
The keyboard is not ideal for extended typing, but neither, to my mind, is the cramped keyboard on a netbook. You can get a bluetooth keyboard for those times that you need to type a lot, and still come out with a smaller package than some full-size laptops. That's what I've done, but I generally find that after a day of riding, sitting down and typing an essay is not a high priority. I still expect to carry the keyboard on some trips, but I haven't yet used it enough to justify it's place in my luggage.
Ultimately it depends on what you want to do. For me, the iPad is the ultimate travel resource for easily pinpointing my location, where I want to be, and what I can find in between. And it keeps me in touch with The World At Large at the same time. I know that a laptop can technically do more, but the ease of use of the iPad makes it easy to grab and check without even dismounting the bike, something I have a hard time imagining with a laptop.
So if the iPad is capable of doing what you need, I think it's a great travel tool. I highly recommend springing for the 3G model even if you plan on using wifi only. The GPS really enhances the functionality of the iPad when on the road. I used to want a laptop to supplement my desktop, but since getting the iPad, I can only imagine a few very specific scenarios where I'd want a laptop over my iPad, and so many more scenarios where the reverse is true.