From CJ's assessment:
...These are all welcome additions to Microsoft’s mobile OS, which until now seemed like it was way behind iOS and Android in terms of features. With Windows Phone 8.1, Microsoft has finally caught up, and in some specific ways even managed to exceed rival mobile platforms.
At the end of the day, there’s much to like about Windows Phone 8.1, and it’s actually quite impressive what Microsoft has brought to the table. Sure, it took them a while, but Microsoft has finally closed the gap, and it no longer feels like a platform that is trailing behind by years. For the first time, Windows Phone finally feels like a *finished* OS that is no longer in Beta. Now all Microsoft needs to do, is get the app ecosystem sorted.
Windows Phone 8.1 is available out of the box with new Lumia phones like the Nokia Lumia 630 and the Nokia Lumia 930, and will be available as an over-the-air update for all Windows Phone 8 Lumias, including the Lumia 520, 525, 620, 625, 720, 820, 920, 925, 1020 and 1520.
You can read the whole review here. CJ goes over all the major aspects of functionality and likes what he sees.
Windows Phone 8.1 is becoming more and more of a necessity in the Windows Phone world in recent weeks, with new software such as Files and Movie Moments requiring 8.1, for example. Happily, most enthusiasts reading AAWP will already be on the Developer Preview and, as CJ quotes, it's only a few weeks away from general availability as an over the air update now.