As a Microsoft developer with an MSDN Ultimate subscription, I've always enjoyed the privilege of early access to RTM (release to manufacturing) and RC (release candidate) builds of most Microsoft software, including Visual Studio and Windows. Earlier this month, when Windows 8.1 was officially announced as hitting RTM it was also announced that MSDN subscribers would not be getting early access.
That announcement rubbed many developers and other tech "press" types the wrong way. (Personally, it didn't really bother me. Yes, I was certainly annoyed that I would have to wait another month until I could install Windows 8.1 on my machines, but it wasn't the end of the world as I knew it and I didn't cry foul and say that the sky was falling.) However, the message that Microsoft heard, and heard very clearly, was that this change in the way RTM bits were released was a mistake and, as of today (September 9, 2013) the RTM builds for Windows 8.1, Windows 8.1 Pro, Windows Server 2012 R2 and Visual Studio 2013 RC are available to MSDN subscribers.
I have been using Windows 8.1 Pro on 3 different devices (desktop/non-touch, laptop/non-touch, and Samsung tablet) since the preview release was available. Overall, I like it. The changes to the start screen and the customization options are nice. I could do without the Windows "button" on the taskbar again (I never really missed it in Windows 8) but it's not as annoying to have it again as I thought it would be. There are a lot of other differences (improvements, new features, etc.) that definitely make this a "must have" upgrade. Since it's also a free upgrade through the Windows Store (when it becomes generally available next month), there is no reason for current Windows 8 users not to upgrade. If you've been holding out on upgrading to Windows 8 you should really consider the upgrade to Windows 8.1.
From my perspective (and given the changes that are happening in Microsoft right now as a result of the reorganization and the Nokia acquisition), it's clear that the "modern" user interface (first introduced by Windows Phone) is here to stay...and that's actually a good thing.