Pocket Informant's great. Fast, easy navigation, timebars, and todos added into the calendar database. It's still a fairly new app, and calendar programs are complex processes that simply take time to streamline, but the developers have an excellent track record on other platforms, and they've continually updated this app to add in new features and streamline it. That being said, there will be slightly slower processing speeds for some of the editting and syncing processes till the developer locates and fixes the bug or endless loop. I've personally found it to be awesome though, it's fast enough for my needs (aside from a version ago when deleting a recurring appointment took at times twenty seconds to process, it now just takes a few seconds). Don't mind the icon, I think it's ugly, but there's a new and professionally made icon that looks great coming in the next update (probably two weeks later).
The other app that I used before Pocket Informant came out was Saisuke. It's had time to have processes streamlined, and it's copy and paste of events is VERY efficient.
I prefer Pocket Informant though cause it has timebars, and I can see events and todos all within the same app (I don't have a lot of todos, so I didn't open up my Todo app often, and missed doing a couple important things).
Pocket Informant and Saisuke both sync to Google calendars, and you can set up the iPhone calendar app to sync with the same Google calendars. The inbuilt calendar is great for notifying you of upcoming events, something no app can do yet while the app is not open (Pocket Informant will have push notifications coming in an update due sometime this summer).