I haven't upgraded to Sierra yet because I am not dying to, I like El Capitan a lot and it's working perfectly for me. I've been using it since April and there hasn't been one issue yet, of any kind. I have experienced exactly zero problems with it and that is something that I am very happy about. For those who have upgraded, are you happy you did so? Are you experiencing any differences in user experience? And would you recommend other 5K users to upgrade too? One of the reasons I haven't upgraded is I'm not sure if I'm going to run into some unforeseen glitch or error, my machine is the 3.3 i5 m395 model so I am not concerned that there will be difficulty running it, but who knows if there are issues on the software side of things that still need to be worked out.