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Depending on your priorities, there are a number of apps you could try instead of the native calendar:

http://www.iphoneness.com/iphone-apps/10-best-calendar-apps-for-iphone/#idc-cover

http://itunes.apple.com/us/app/touchtodo/id299665280?mt=8

Right now I just use Google calendar synced with my iPhone calendar; still looking for the perfect setup. I was tempted by Pocket Informant but the price in combination with many negative reviews, and no free Lite version to check it out myself, put me off so far. That said, I had a look on their forums a while ago and what they have planned for the future sounds interesting.

I use Toodledo mostly as backup in case something happens to my phone or 2Do. I’d hate having to manually re-enter all my tasks! And if I have a lot of tasks to enter I tend to do that via Toodledo, rather than 2Do, since it means that I can use a proper sized keyboard.

If you’re looking for a more integrated solution, perhaps Producteev would work for you? It’s free if you go for the basic version and you can use it for basic GTD. Bobby has written a great article about it here:

https://www.40tech.com/2010/09/03/gtd-in-producteev-utilizing-multiple-workspaces/

I’m not using it because I prefer local (push) alerts on my phone over email alerts, but that’s a personal preference. I travel a lot internationally and email alerts require data roaming, which can run up to be very costly. But that’s just me. :)