They redesigned the entire OS in like 6 months. It's Ive's first major foray in to software design. I think we can cut Apple a little slack here. iOS 7 might not be perfect but it will get refined and improved upon. I suppose Apple could have waited until iOS 8 for the full redesign but then people would have complained about the UI being stale, Apple is doomed etc. I'm not judging iOS 7 until its installed on my phone and I get the opportunity to actually use it.
The entire OS is not a 6 months work. It can goes as far as 2 years ago.
Source: WWDC developer videos (I forgot which section)