When using HDMI mirroring there was input lag.
There are framerate hitches while using the UI and that is my main issue. It does run at 60 fps, but it certainly doesn't maintain it. I found that, while swiping through pages, the first 20% and the last 20% of the animation was 60 fps, but during the rest of the animation it drops every time. It simply doesn't feel smooth.
Going into various menus, lists, and web pages produces the same issue.
I know very well that a lot of people simply do not or cannot see these hitches and don't find them bothersome, and that's fine, but it's very noticeable to me. It doesn't matter how fast the hardware under the hood is when the results are like this.
As for Jellybean, I don't know if those phones were using it or not (probably not), but I HAVE used Jellybean extensively on an Asus Transformer Prime Infinity just the other day. The same problems are everywhere. Now, granted, that tablet has a 1080p display so I suspect keeping the framerate high is limited by this factor, but still, it was disappointing.
At its base, yes, it's 60 fps...but it drops constantly while using it.
Now, if you root the phone and eliminate Samsung's software I suppose things could change. Shouldn't have to do that to achieve smooth performance.