We recently traveled to London and Rome and I bought the app for both cities. I also bought two guides which helped me plan the overall trip, but what this app offered was the ability to download maps offline and find great little restaurants, bars, pubs and places of interest based on a location youre at.
For example, we would go to the tower of London, and use this app to find reviews for restaurants in the area. We did the same in Rome, plan to visit a landmark using the guides we bought and then use this app to find other things lose to our location.
Not having to be connected to the Internet is great and being able to finds almost anything based on your location is even better.
In London, we also found the Underground Map a great resource and again, we could download it to view without Internet connection.
Boerseun.com about London Travel Guide Offline