My search for the perfect software task management solution has been going on for a long time, and I think I've finally found the answer: Remember the Milk for iPhone (iTunes link). I've tried almost all of the big names in task managers: iGTD, Things, OmniFocus, and even Leopard's built-in todo functionality. None of them have met my needs.
For me, a task list needs to be ubiquitously available, and very quick and easy to interact with. For me, that means I need a native iPhone app, which immediately eliminates iGTD and Leopard's To Do list, since there is no iPhone equivalent. That leaves me with Things, OmniFocus, and RTM, all of which now have native iPhone apps.
One of my requirements is that my tasks synchronize to a web server, so that I don't need to have my laptop turned on to synchronize my tasks to my iPhone. This eliminated Things as a contender for me, since it currently only supports direct synchronizing over the same wireless network. OmniFocus had therefore been my de-facto task management tool due to its ability to synchronize both the Mac and iPhone versions to either MobileMe, or your own web server using WebDAV. The main problem I've had with OmniFocus on the iPhone is how incredibly slow it is to start up and synchronize. A recent update attempted to address this issue by allowing new tasks to be added while the synchronization was occurring, but it's still just not quick enough.
Remember the Milk for iPhone synchronizes to RTM's web service, making it available from anywhere by default. And it does so faster than any other app I've used. Launching RTM for iPhone takes only a second or two, and you can immediately start interacting with it. The automatic sync it does upon opening takes only another second or two, particluarly when using a WiFi or 3G connection. Edge is only marginally slower.
Once you're in RTM for iPhone, you can see that the web developers behind it decided to go back to the drawing board and approach this app as an iPhone app. Nothing about it feels like a web app, and in fact it feels so naturally iPhone-like that it's almost surprising. Gestures like swiping across a task to complete it work as you would assume they would. Date and time pickers use the standard wheel controls, but with an added polish: if you're simply looking to give a task a date, you can use the Basic date picker, which gives you Never, Today, Tomorrow, and then the next week listed out. Or you can switch to the Custom interface, and type in your date and time manually. RTM for iPhone remembers your choice, so the next time you view a date and time picker, it will show you the last view that you used.
Regarding task dates, Remember the Milk in general handles postponing tasks more elegantly than any other task management application I've used, and the iPhone version is no different. I can simply hit the Edit button, check off as many tasks as I like, and press the Postpone button to set the date for these tasks to the next day. Since I often think I can accomplish more in a day than is really reasonable, I will often have twenty items on my list for a day, but only complete seven. Postponing thirteen items in RTM tasks only a moment, whereas it was taking me a number of minutes in OmniFocus for iPhone.
I also like that managing your various lists works the same in RTM's iPhone app as it does on their web service. You have full control to manage and update your task list no matter where you are.
There's no doubt that Things is beautiful, and OmniFocus is powerful, but the Remember the Milk web application for me is just as functional as those other two apps, but available from any computer. Even better, Remember the Milk has a wonderful Firefox add-in for Gmail, and now with Gmail's ability to add gadgets has a gadget verison. This means that I now have access to my tasks on any computer when I log into Gmail. To get a task into RTM from an email (99% of my tasks begin life as emails) is a snap on any platform; I just email it to the private email address that is associated with my RTM account, and optionally add in some fields to allow the task to be pre-filed. It then shows up in my RTM Inbox list, or the list I specify.
The introduction of Remember the Milk for iPhone has finally brought my constant search for my ideal task management platform to an end. Maybe it will for you too. The iPhone app is free, but you must have a Remember the Milk Pro account to use it, which costs $25/year. If you haven't done so already, you can try RTM Pro for free for 15 days and give RTM for iPhone a whirl.
As an aside, I wanted to point out that RTM is maybe the only company I know of that is earning 100% of the revenue for their iPhone app, rather than the 70% that Apple offers, and even better is able to offer a no-cost trial version. And amazingly, they aren't doing anything that Apple can get angry about. Clearly RTM had pre-existing infrastructure that allowed them to release an iPhone app using this model, but I imagine that other developers that are building online infrastructures will be looking to RTM's model as a way to ensure they capture as much of the revenue as possible from their next iPhone application.