Given that I had almost 14 GB in my monthly internet allotment remaining, I decided to try to download the latest operating system and install it on a laptop computer that I've had since around the first of 2013.
In November 2014, I had Apple install 10.9.5 on my machine because I didn't have enough data available to download the operating system. I also remember all the pain of downloading 10.8 and the fixes and having to jump through hoops to get it to work, and then, I changed to an SSD and I couldn't just copy the operating system as in the old days. Gnashing of teeth was part of the installation process.
The time for the download was not horrible, although my Verizon mobile hotspot has some very variable service lately. Once downloaded, there were a couple of items to confirm and then, it restarted to actually perform the installation.
At some point, it asked me about iCloud--and took a very long time to complete. It also did this for a secondary user on the machine, but didn't seem to take nearly as long.
That was the big deal.
Performance is much better, although the interface is so flat (with sharp edges) that I suspect that the next major version will have concave features. It feels a bit as though children were tasked with the re-design.
Maybe, it's just that 10.9.5 was really awful--10.7.x was, and 10.8.x was also. In fact, the last time I really felt good about performance was 10.4.10 on the PowerBook G4. However, 10.11.3 seems a strong improvement.
I haven't seen a problem, other than having to update all of my iWork applications. Why they are so close to the operating system, I don't know but it feels like when Microsoft was using hidden APIs to "enhance" Microsoft Office since the compiler couldn't fix the code execution to be faster. Oh, and my 14 GB remainder is suddenly about 3GB.
Update 2016.04.05: There is a huge drawback that most people won't likely see. When I try to transfer over 500 photo files from SD cards using my USB 3.0 enabled reader, the Finder seems confused.
It seems to create ghostly icons of the files in the folder but never seems to actually copy the files. It doesn't report any progress. I end up having to re-launch Finder, delete the files' icons--or whatever it takes, depending on my level of frustration.
There is also a problem while transferring around 400 photo files, but this has been lessened by 10.11.4. Progress stalled quite a lot with 10.11.3 and shaking the Copy dialog box seemed to help. It's smoother now but once it's stuck, nothing helps. I could try using the inbuilt SD Card slot but it didn't seem to work in previous releases, so that's not a good idea.
Why is Apple having USB problems after all this time? I've thinking that the all-in-one iMac from the 1990s had severe USB problems but that was so long ago. Then again, every major release (including iOS) seems to have WiFi problems.
Why isn't Windows 10 such a problem? I'm surprised that things work so smoothly. I suppose Apple's problems could be related to their lack of procedures.
Update 2016.07.20: Up to 10.11.6 now. Finder has not been updated but seems somewhat more reliable, perhaps because of background processes. I've lost faith in Apple, but then, my faith in them has been diminishing since 10.4.10. 10.5.x for PowerPC was terribly broken and several applications stopped working. 10.6.x for Intel was acceptable finally, with the white 2.13 GHz MacBook. My Mid-2012 MacBook Pro is the only one I still have that can be updated. It was awful on 10.7.x and 10.8.x but 10.9.x was reliable.
The good thing with 10.11.x is that the App Store works reliably.