Just an update here after my original post this morning. I've been trying to use this new UI all day, and the more I use it, the more I hate it. And I'm constantly afraid I'm going to accidentally perform an action I didn't intend. One issue I ran into today, was code that I had deleted, was showing up in the UI. I could have sworn I deleted it and was wondering why it was appearing as though it had been added. Looking in the SourceTree app, the lines in question were clearly marked in Red. But the Xcode UI instead shows it as a faint background, the exact issue that I see @YukonJJR describing above. Maddening.
Unlike the original side-by-side editor (which existed as such for years and no one complained about), this new version as far as I can tell is no longer a live editor either. I can't simply go and fix a typo in a comment right there as I'm reviewing the diff, as I normally would with the functional commit interface in prior versions of Xcode.
I also ran into many of the issues @keithburgoyne described, terrible scrolling, not showing files properly, etc. And its not like I was making major changes.
I just don't understand these decisions and how something so shoddy and questionable like this can be released. Especially when there are plenty of other areas in Xcode that are buggy that need fixing (as far as I can tell, none of the bugs I reported in the virtual labs this year at WWDC to the team seem to have been fixed). But yet, the team had the time to "fix" something that wasn't broken.
And its not just the dev teams, some of these product decisions like the "Fine Woven" case seem to be becoming a debacle that should have been spotted a mile away. Like what some friends repeatedly say to me as of late, the quality of everything has just gone downhill, and so have the standards.
I went and filed a Radar anyway requesting they revert back (or at least provide a setting to enable the previous version). Thats really all I can do that this point, and hope others do the same.