I have been using 8.1 for about a year now.
A little quirky to get used to the interface but once you do - I prefer it. I have had no performance issues at all.
I recently put a new PC in our business for my wife with 8.1 on it. I offered to 'turn it back' to 7 for her. Her response was "nope, I will work it out". LOL, 4 weeks in and she loves it.
I often think the people that cry about it simply have not spent the time to learn how to use it. Of course this is the Internet and I will now be flamed but such is life. 
My Wife was the opposite. She gave it six weeks, but then brought the laptop to me and told me she did not want it back if she had to keep using Windows 8.1. Her question to me, "Why would anyone design an operating system to get in the way of running applications?".
She got tired of constantly having to Google for help on how to do things. It was not a matter of learning how to use it, it was that it required so much more effort to do anything than before that got her going.
Her other favorite rant, "If I wanted to use a mobile phone to run my apps, I would use the damn mobile phone!". This in reference to how you have to drag the cursor off screen to get to menus.
I have to concur with her. Windows 8.1 seems more mobile phone oriented than desktop oriented, but even then it fails. It just seems to be an operating system looking for an identity and cannot find one.
She also got irritated about having to buy other utilities (a decent media player) which are already bundled in Windows 7. Seems Microsoft took the opportunity to rid itself of anything it had to pay license fees for, while also raising the price of the operating system. Good for the bottom line, not so good for the end use.
