I certainly think so – it's one of my favourite apps. I think it offers the best on-device task management interface, although, like Greg says, there are some syncing problems. However, I've found the app to be so good that I'm really doing all of my ToDos on the iPhone now.

The thing that really separates it from other apps is how fast it is. It's not the fastest to load up, but all of the steps you take inside the app feel like they've been thought out and tapped out well in advance. So if you're adding a new task, you can just tap the new task button, and depending on which calendar (category) you're in, you'll already have some information keyed in, and all you have to do is type a title and press enter. The keyboard even pops up automatically, so you don't have to tap inside of any dialogue boxes.

I've used Things, ToDo, RTM, and a couple of others, and I still like 2Do the best.