El Capitan is the second major upgrade we had from Apple at a free price tag. With Maverick they proved this concept and they received all kind of appraisals and cheers.
Now into the new features. I loved the multi task resize (side by side) of the windows. It's something that you would think that it should be there since the beginning and yet it was missing all those years. I used to purchase a product to solve those kind of perks for me but it's nice to finally have it bundled in my vanilla OS, good move Apple.
Mission control also received some tweaks and they are immediately visible as soon as you log in. You will notice that your old view of your desktops when you switch your applications (F3 key) is revamped, now it shows the desktops names only without their icons or small previews on top, that saved a ton of space and you can toggle them as soon as you hover them with your mouse, again another small changes that improve a lot your visual area (specially with small screens like MBP 13') and that should be part of the core system a long time ago but it's finally here (which means Apple still knows how to do nice things for free).
My most used feature is probably spotlight, it got huge upgrades now being able to search all kind of things so instead of opening a new tab in my browser (or open a browser at all) I got very used to just simply invoke spotlight and start typing what I want. If you integrate that with your notes to take screenshots or websites and store them visually it's a perfect match.
They also made some improvements to the video game department (metal), but as a serious gamer guy I would never use a Mac to game except when I'm out of town and have no other option in which case why even bother?
UI wise it looks slightly better, the fonts on a retina display are crisp and beautiful, their new wallpapers are also designed to get full advantage of those screens and in general it feels the old good and fast reliable OS you always trust and need.
If you don't have a reason to hold back (like an incompatible program not being ported yet) just upgrade and enjoy it, it's free!
Some of their UI changes have being very contrary to what they used to do, I kinda miss their old plastic bubble look. Their icons are not the best either, their colors are way too shiny for a professional look IMO.
Not a huge fan of safari or maps either but I could see their use.
Make sure all your software is ready for upgrade, I've known of a couple incompatible software suites out there but for most people that's not a problem and they should enjoy it right away. As a developer it's a must, you get the new XCode versions and upgrades.
I'm a software developer and I have currently 2 mac book pros with client code in each of them, so I needed a very robust reliable system and OS X fit my needs perfectly, you have the great console from UNIX (linux) and yet a very beautiful and easy to use UI which linux lacks.