When I had mine, it was fairly smooth but sometimes would lock up and stutter. Once I was showing a friend the device, and it was super stuttery to the point my friend was like, you paid money for this? And there were other times when it just locked up for s few seconds, but not always. Enough to be annoying.
Overall it did tend to work well, I could stream music and do GPS and BT and no issues. Sometimes the keyboard would lag. It was not as smooth as my iPhone even at the best of times, but by no means a bad experience. I was more than happy to keep it, aside from the battery problems going on.
I had an s3 and hatred TouchWiz, but was impressed with how much it improved in the interim.
With all the worry though about the resale values, use on planes, and my personal fear that Samsung won't continue to support due to less people buying one, I may not get it once it comes out again... Shame, I loved it while I had it. But mine got very hot a few times, so didn't feel safe with it. And now my confidence is low.