We do tour. Our tours are typically week long, self supported credit card tours (no camping or cooking, we stay in B&B's or hotels and eat in restaurants). We tour with a Garmin Edge 705, an iPhone, and a Mac laptop. We use the Garmin for navigation, but will use Google maps on the iPhone to confirm or search for alternatives. I would not rely solely on the iPad for navigation due to signal and battery life (we have a small extra external battery for the Garmin that uses AA batteries so have almost unlimited life).
I was interested in replacing the laptop with an iPad for the reduced weight. We keep a journal/blog of our tours and use Apple's iWeb to do so. Unfortunately there is not (yet) an iWeb app for the iPad. We could switch to another blogging platform but we haven't wanted to do so.
For US tours, the iPad would be great for email, internet access, blogging (if you don't use iWeb) etc. For international tours, the iPad would be costly. The international roaming charges are the same as for the iPhone - extremely expensive.