Since everyone is ashamed to write your opinion, I'll break the ice for you and write my first experience on Win 8.1 on HTC 8S
So, since I came from android to WP and was get used on android, I first noticed how on WP you can't toggle notification bar and see shortcuts for wifi, mobile networks... In win 8.1 is that thing fixed. You can easily slide down from upper side of screen down and you'll get notification screen and shortcuts. It's really easy to set it up and you don't have to do boring steps, open wifi menu, turn off wifi and it automatically it connects to network. On win 8.1 when you scroll down notification bar, you can just tap on wifi icon and it automatically connects to saved network, just like on android. One big + for win 8.1 in compare with win 8. Also, as I said how I came from android. Android has a big problem with lag. If you install bunch of apps, your phone will be slow and will lag like it's 10 years old, unbelievable. Just can't get over it. Since I moved to WP I have not experienced any lag what's really awesome. Also one big + for WP.
Battery life on win 8.1 for me is really great. I don't use my phone a lot. Maybe few times on day for about 10-20 mins. In whole day it would be 1-2h and my day ends with only 20% less battery. Which is really great for me. I don't have to charge every day phone. I charge it every 3 days.
But it would not be all so perfect, I noticed one bug. It's not a big deal. Battery saver. Battery saver should be app that saves battery on your phone, but what a hell is it doing on this phone? It turns off notifications, sync and apps for what? It doesn't really saves battery when it's turned on. In a fact even more consuming battery. Wtf? Also, I left Battery saver turned on all the time and charging was also messed up. My phone charged to 100% in few hours and few mins later it drops from 100% to 84-83%. I really don't know what's the problem with that app. Also, my advice is that you forget about that battery saver. Just leave it off, you'll have even more battery juice when app is off than when it's on.
So, yes. This is my experience on 8.1 with HTC 8s. I'm really satisfied with this phone, even more. Really surprised with windows phone. I really recommend Windows Phone to everyone who's annoyed with lag and bad battery life on other phones.