When the iPhone first came out in 2007 I wanted one. It did not have a todo list app which was something I really wanted so I waited. When the iPhone 3G came out, with the ability for 3rd party apps and such I knew I had to have one. The people that make Pocket Informant announced they would start developing an iPhone version. I loved Pocket Informant on my Windows Mobile phone so I got the iPhone 3G a few days after the 3G was launched (7/15/08). It took awhile for Pocket Informant to be released, it was released in March 09. Before that date I tried a few task apps, ToDo (Appigo) was the best of the lot. PI did have it's problems when first released but through a few upgrades most of those issues are now gone. Up until the last release I kept ToDo from Appigo on my iPhone as it does have some nice features that are not yet in Pocket Informant, but I find my self using it les and less and quite honestly since the laest update to Pocket Informant I haven't even opened ToDo, but I have not yet deeted it from my iPhone.
One thing I noticed about task apps on the iPhone is that they are mostly GTD (Getting Things Done) based apps. I'm a long time Franklin Covey user and Pocket Informant is one of a few apps that actually gives me the choice of using the Franklin Covey method of task management. So for me, that alone is a reason that I use Pocket Informant more. The integration of calendars is also a great feature. The Pocket Informant calendar is a thousand times better than the one Apple built into the Phone. Basically Pocket Informant has replaced 2 apps n the iPhone, Calendar and ToDo. A future update may replace a 3rd app that I am using when they add note ability so it may replace Notebook (also by Appigo).
As far as sync goes, Pocket Informnat syncs with Google Calendar and Toodledo. I can then sync my Google Calendar with Outlook via gSyncit and it works great. This also allows me to (as a backup) have MobileMe sync the Outlook calendar with the Apple Calendar on the iPhone. There is a sync solution to sync Toodledo tasks with Outlook, I tried that solution but didn't care for it so I still enter the tasks in Pocket Informant (either on the iPhone or through Toodledo on the PC) and Outlook manually. May not be a solution for all, but for me it works. At first I didn't think I'd like having to go through the web based apps (Google Calendar and Toodledo), but now that I have been using them for awhile I really like it. I can now access and update my calendars and task lists from anywhere I have access to a PC as well as from my iPhone.
Franklin Covey also has a task program (FCMobilelife Tasks), it's OK, and has gotten better over time and through some updates, but it does not sync to anything yet. They also just released FCMobilelife Goals which is great Goals tracker. Franklin Covey offers a product for Windows Mobile and Blackberry called FCMobilelife which syncs several apps together and I beleive they are slowly bringing this to iPhone one module at a time. I think once the other modules are developed they will bring the full Mobilelife service to the iPhone as well.