I have 'upgrade' to windows 8.1 for around one week now. The windows 8.1 are still in beta stage though and I expected to find some teething issues, I will point out some of these issues that I have met during my use of the windows 8.1.
Pro:
1) Cortana... almost the same as Android Siries. I have a brief encounter with this system and loved it alot. However the biggest problem now was that it was not available to market outside US. So I have to manually change my phone setting to US setting (language and region) before being allowed to use the Cortana. Which is a real bummer.
2) Another feature that I really like is the information bar that appeared at the top of the LCD. It somehow shared certain features with the Android OS and this feature is something that I have wished for when I used the windows phone.
3) Third feature that I like was that it differentiate the volume control from notification and other media, so you can individually change the volume level for ringtone, notification and media playing (like games etc). In the past, there is no way you can do that, so when I tune down my game or even silence it, or play my music, the phone's ringtone was also muted for volume down.
4) The forth feature that I loved was that I can finally type messages in some of the forums that I am in. In the past, this was not possible, I don't know the rationale behind this, but it certainly is a welcoming change.
There are other few improvement, but to me these are minor and I will update them in a later date.
Con:
1) Battery... this is a disturbing degrade. In the past, I can get through one day of use (reading news, access to facebook and surf a bit of net, play some games, watch a movie, listen to music) and still get around 70% of juice left at night. However, with the new update, I find that I get much lesser juice. I didn't watch a movie to listen to music, just read the news and play a couple of games and I get to the night, it has less than 50% of juice left. Luckily my 1520 had a massive battery capacity, so it is still alright.
2) In a couple of occasion, my 1520 become very warm... this happen at around the top area at the rear cab of the phone around the camera area (note that I am not using the camera). This normally happen after one or two games and reading of the newspaper. And in a couple of occasions, I am force to switch off the phone for a while and wait for the phone to cool down before switching it on again.
3) There is one single time when the phone at the front screen (at the tiles) was 'hang' and kept showing the black screen and the little word 'loading'. I was force to switch the phone off (and luckily I am able to do it).
4) Some of the programs (eg. Facebook) can no longer to switched off by pressing the 'back' button. You need to turn off the program manually. And that is a bummer. I noticed that I have forgotten to do that in some occasions and it drain my battery faster, as well as cause some heat issue with the phone.
Conclusion
The windows 8.1 is a very fun OS to use and had definitely answered to some of the lacking in its previous OS. But there are still quite a number of issues. However, do note that this is a beta version and so I would expect that windows solved all the issues by the time the real version was being released.
(Note: you cannot convert back to windows 8 if you have upgraded to the 8.1... so do take some time and look through the cons. If they are alright with you, then do the upgrade, or else... wait for the official version of the 8.1 to be released.)
