Our team built a very similar product to Brewster, which shared many of the same features. We discovered a lot of issues with building an iOS address book repla

Our team built a very similar product to Brewster, which shared many of the same features. We discovered a lot of issues with building an iOS address book replacement, and we never launched the product for those reasons. 1) You cannot access the recent calls list. Most people don't use their address book to keep in touch, but rather use their recent calls list. Not being able to access that list means that a huge number of people can't replace the default address book with your product. 2) You cannot be the default. Whenever I make a call, I am kicked back into the built-in Phone.app. This is incredibly confusing for users. 3) It's a momentous performance challenge. When users open the default Phone app, their contacts appear instantaneously. If you want to replace the default, you need to be sure that your address book does whatever it needs to insanely fast. We managed to get the process of syncing an address book down to seconds, but Brewster has unfortunately taken nearly an hour for me. Many, many people will drop out at this point and think the app is broken. 4) It's an enterprise product. Most people's address books are incredibly tiny and include bad data (nicknames, just first names, no names at all), which makes it very hard for a service to provide any type of value add. The only place where you can do an excellent job is for people with thousands of contacts. 5) Privacy concerns. For a product like this to have a great value add, you need to provide the service with a ton of personal data. You need my Facebook, you need my email, you need my address book and much more. Many folks will see this and drop out of the app right then and there. It might be possible to get around these hurdles, but Brewster has a long way to go.