I've never been a fan of single-window IDEs, going all the way back to Visual C++ 6, and I don't think XCode 4 is going to be the one to change my mind. And god damn it, all the keyboard shortcuts have changed! This is going to be painful.
It's clear they've put a lot of work into it. It looks slick, and the organization of the menu and commands is much more thoughtful. The biggest problem with current version of XCode is that it's very difficult for newcomers to learn, and this new version will go a long way to correct that.
But none of that matters to me. I can't touch this software until I'm forced to use it, because when that happens I'm going to have to un-learn everything I currently know, start from scratch, learn all new habits, etc. Can't say I'm looking forward to it.