Why I'm Not Upgrading My iPhone To iOS 7 Right Away
First of all, I haven't backed up my phone recently. Before you update, please be sure you do this. Apple's update process has become pretty foolproof, but never doubt mankind's ability to break stuff - backing up your phone takes five minutes and will save you a world of hurt if something goes wrong.
Secondly, not all apps are optimized yet to fit in with the look and feel of iOS 7. There are design and interface elements that can come together to make an app feel like it's a native part of the phone, built-in and supposed to be there. But implementing the changes that iOS 7 calls for obviously takes time. And there hasn't been enough of it yet.
Finally, there will very likely be another update in a couple weeks that will iron out any wrinkles that went unnoticed in Cupertino the first time around. I'd simply prefer to go straight to this updated update.
One other reason to skip the upgrade for a little bit is that your device might not run iOS 7 very well (if at all). Anything before an iPhone 4 and you're simply ineligible for the update. iPhone 4 owners may want to skip the upgrade altogether, as the phone's hardware is a little aged at this point.