I recently spent a week in Portugal for EWSN'10 and spent a few days in Lisbon and Porto on either end of the conference. I decided this time to go entirely paperless -- that is, not take a paper guidebook. Rather, I was going to rely entirely on my iPhone for all of the travel information. As an experiment it was largely successful, with some caveats.
Normally I take a Rough Guide or Lonely Planet guidebook with me when I travel, but this has two big disadvantages. First, I have to lug the book around wherever I go, which usually means also having a bag or something else just to carry the book when I'm out on the town. Second, having the guidebook out in a bar, restaurant, or on the street immediately pegs you as a tourist and I hate being so conspicuous. I'm all about blending in, as the picture on the right should make absolutely clear. (Pop quiz: Which one is me? Hint: I don't smoke.)
This time, I decided to rely on the iPhone Kindle app and bought the Rough Guide to Portugal for Kindle. So the entire text of the book was in my pocket at all times, and reading the book on the iPhone just makes me look like another cell-phone-obsessed tech junkie, which is fine by me. (At least in most places that I travel, although I have been to some pretty dodgy places where messing with an iPhone in public is likely to garner some unwanted attention.)
The big disappointment was that the resolution of the maps in the Kindle Rough Guide is not good enough to actually read the street names and markers -- even when zooming in on the map. I am not sure if this is unique to the iPhone or whether I'd have the same problem on a proper Kindle (I don't have one so I can't tell). I can say it isn't a problem when using a paper guidebook. So I could not really use the maps in the guidebook at all.
Of course, Google Maps is great on the iPhone and the GPS feature is a huge help when you're trying to get your bearings. However, this requires use of your data plan, which is expensive overseas. I bought a 50MB international data add-on which costs about $60. Other than having to monitor my usage it was a pretty good investment, though I really wish it were not necessary.
There are a couple of iPhone apps allowing you to download maps for offline viewing, including OffMaps (and quite a few rip off apps that simply take the same data and package it for a single city and sell you that alone for 99 cents.) They also permit use of GPS without incurring data charges. Unfortunately, they use free map sources that are much less accurate and complete than Google Maps -- the map for Coimbra was just terrible and only showed a couple of major highways, and none of the smaller back streets of the city. So the quality varies a lot.
The best iPhone app by far was the Lonely Planet Lisbon City Guide, which includes a great map with all of the restuarants, bars, etc. listed and linked to a little page telling you about the place with its hours. You can even search the guide, unlike the Kindle app which has no search capability. It's pretty terse but for a few days in a city was more than adequate. I also like how Web links can be tapped directly -- in case you want to dip into your data plan, say to check out the website for a restaurant or hotel -- the same is true in the Kindle e-books as well.
The final caveat was the limited battery life of the iPhone. The Kindle app does not eat a lot of power (I've read for hours on it with hardly a dent in the battery) but use of the GPS is pretty energy-intensive. While traveling I was using the iPhone a lot more than I usually do, and found that by late afternoon or early evening I was getting into dangerous territory, necessitating a quick recharge at the hotel if I was going to last the evening. Fortunately this coincides with my usual tourist siesta so it was not a problem.
If it were not for the poor resolution of the maps in Kindle Rough Guide, this combination of apps would have been an ideal solution for travel without a paper guidebook. If they can just fix that I'm ready to conquer the world without a book.