I've been running 8.1 for the past couple days on my Lumia 920. Overall, I am impressed.

Good stuff in 8.1:

- The new calendar user interface, as Anand points out in his review, is more polished than it was in 8. This was needed since the Calendar in 8 was a bit amateurish in its appearance. If you looked really close, you could see that it used pseudo-Latin words on the days in the month view to indicate you had appointments on those days. But the words weren't the appointment names!

- Six columns of live tiles (3 of medium size) makes the home screen information-dense. I have headlines, updates, the latest movie news, calendar items, weather, TSLA's price ($192?? Maybe time to buy more?), e-mails, and a bunch of other nonsense flipping around and dancing. It's a bit chaotic but overall I like this a whole lot more than what I remember from when I used Android and before that, iOS.

- The signal strength, wifi, and battery life indicators are now always on the home screen. You no longer need to tap the top of the screen to see these. I liked the minimalism they were aiming for by making these slip away after a moment in 8. But I can see why they decided to bring them back as permanent fixtures. Luckily, they still hide in apps, giving maximal screen real-estate to productivity.

- Finally, an official podcast application that accepts URLs. In 8, I wasn't able to subscribe to a podcast via URL, so if the podcast I wanted wasn't in Microsoft's library, a third-party app was needed.

- Accessories such as Data Sense, which gives a breakdown of data utilization, and Storage Sense, which shows how storage space is used. These types of accessory are old news for Android and iOS, but were oddly lacking on WP. I had been using a horrible proprietary AT&T app to monitor data usage previously.

- The notifications ("Action Center") user interface is nice. Very Android-ish.

- Cortana is a cute addition. Finally having a voice assistant app confirms the suspicion I had when Siri arrived on iOS: I don't really have much of a use case for this. I suppose the next time I need directions somewhere, this will be nice. The sparkle of more promise is in Cortana's potential to interface with applications. If, for example, Cortana can use GroupMe (my messaging application of choice) to send messages in addition to SMS, I could find myself using that more frequently. But I don't use a lot of SMS, so Cortana's messaging feature is of limited value to me.

Bad stuff:

- I've had the Calendar application and the music player each crash once. And in both cases, it was a device-reboot style crash. This is a developer build, so I will give Microsoft the benefit of the doubt that the final version will not crash like this.

- Cortana isn't quite living up to its potential. I've not actually used Siri, so I am not saying it's worse than Siri. But I want to see more applications support Cortana, and more smoothly than is done by the few that do integrate today ("Flixster, show latest movies"). For example, I'd like Cortana to just know that I use GroupMe to message my wife and SMS to message someone else. As another example, Twitter doesn't seem to be integrated into Cortana much yet. I think the only integration is Cortana pushing tweets out. Incoming just arrive to the Twitter app and notifications.