I'd like to know more about the pros and cons of using interface builder when developing iPhone/iPad apps.

I've written a fairly complex and customized app that's on the app store right now, but all of the interfaces are hand coded as they are fairly complex. I've customised the navigation and tab bars with backgrounds, table view cells are manually drawn for speed, and some views are complex and scalable with many subviews.

I'm pondering whether or not to start using interface builder but I'm not sure to what extent I'll use it and whether it's worth it at all. Is it quicker? Can things still be easily customised?

Any advice would be most welcome!