[Ryan] Hoover uses calendaring app Sunrise as one example. Mobile email, calendar, and to-do clients, by themselves, are easily replaceable, but having a consistent experience across multiple screens makes them much less likely to be replaced.
This is one main reason of why I’m tied so deeply into Apple hardware/software and 3rd party applications developed for their ecosystem. Take OmniFocus for example… I have the application on my Mac, iPad, and iPhone. They all work together and have similar interfaces and functionality. Due to having applications like this, that are so deeply ingrained in my daily workflows, make it very difficult to replace them and the devices/systems they run on/in.