I haven't toured with the iPad, but many times with the iPhone which shares many characteristics. It gets most of its use when I tour alone and after I've setup camp for the night. What I like is that it does so much in such a small package. Namely, it's an mp3 player, e-book reader, gps, connection to the world (internet radio, e-mail, www, news, phone, Skype), gives weather forecasts, and has a cheap camera. I don't necessarily use it for all of these things, but it does give you an idea of what's possible.
There are many people who tour with a netbook to write and post their travelogue. The iPad or iPhone can also be used for this, although you probably wouldn't want to type at length with the onscreen keyboard.
For micro blogging it's perfect. I've written a bike touring app called TrackMyTour, which was recently updated to support the iPad. It was reviewed on the ACA website just over a year ago: http://blog.adventurecycling.org/2010/02/tourist-20-trackmytour-iphone-app.html
. It's a micro blogging app, which focuses on your location so friends and family can follow your progress on a map. As cool as this is, I'm not certain I would still bring the iPad when I already have the iPhone with me.