Yes I upgraded on my 5 yr old iMac, its not a sea-change like Yosimite was, some changes seem to smooth background things a little, I think its mostly internal. And it seems to work seamlessly with all other apple devices, including any IOS 9 which have also auto updated, including Apple watch. I read it allows steam computer users (PC's) a better interface with IOS devices, but I wouldn't know, like most people who move to the dark side from a PC, i wouldn't move back. Ive just thrown away a 6 year old Laptop cus its so slow in comparison.
Web use is the same but I do have over 40 gigawotsits download so its instant anyway. Don't hesitate Ive always understood that you should always update when Apple asks.
Downsides - none as far as I can see.
