I think I’d give it 7/10, but if I could go back — but knowing what I know now — I’d be waiting to see what the next update brings before upgrading.
I didn’t have any trouble with Mavericks or Yosemite; actually, I’ve pretty nearly always found my systems to be stable, and kernel panics generally so rare that I really can’t think which version was the most problematic. That said, El Cap does seem to be more speedy and efficient. Under Yosemite, this 2010 MBA, with only 2GB of RAM, seemed to be at the limit of usability and I wondered about upgrading, thinking that El Cap might make higher demands on RAM to the point where it would be rather unusable for serious work — it’s also limited to a 256G SSID, and I do need to keep an eye on free space. Actually, as I said, I’ve found El Cap more efficient from that point of view, and even though I’ve installed Memory Cleaner in case of RAM emergency, I’ve never had to use it for that, though I do purge the memory routinely from time to time. That said, I’m clearly not as heavy a user as many on the Forum, so they may find issues that don’t affect me.
However, the biggest frustration for me is the screwing up of the tab system … that right tabs, etc. switch to left tabs on closing or on restarts of the system — I’m not sure which, as the two tend to go together for me. I would hope that they’d solve that for the next update, so I’d wait, as I said above.
Next, a real problem for me is changes to the trackpad system, presumable to make full use of force-touch, or whatever it’s called, and clearly there’s no way that that’s going to go away. I have an “essential tremor” — frustratingly inessential but requiring continual use of strategies to compensate — and under El Cap, the trackpad has become so sensitive — and there doesn’t seem to be any way of reducing the sensitivity — that it is causing enough control problems for me that I’m thinking I’m going to have to go back to using a mouse at all times — that can be problem enough — but it would help when using a mouse if I could disable the trackpad completely, which I don’t think I can. But all of that is a purely personal problem.
As for the UI, as I said in a post some years ago when the subject came up — when Apple largely gave up colour in favour of 50 shades of grey — I’m not too concerned about the colour/non-colour or design of icons, etc; the only change in UI that I ever made was the hack to get rid of the leather-look, skeumorphic design of the address book/contacts … I did hate that enough to do something about it. And my 70-year-old eyes don’t have any trouble with the latest icons; in fact, in Scrivener, I currently generally use 11 point EBGaramond Pro and 100% magnification. My system is minimalist, with generally a bare desktop, with files on it only temporarily, and the only permanent residents of the hidden dock being the finder and the trashcan.
As time goes on, I find I want to do more and more through keyboard shortcuts, so as long as what I’m working on is clear, and I can recognise quickly any icons I do need to use, I’m happy and the more minimalist the UI, the better. However, in that, colour in icons does help, but I’m not going to rail against the dying of the colour … I’ll just get on with it.