I am upgrading to El Capitan on a seven year old MacBook as I write this. I should have a feel for it in about an hour. My old MacBook runs well but slow. The best and most reliable computer I have ever owned.
We have El Cap running on a late 2008 unibody MacBook. Runs better than Yosemite, IMO. I'm surprised that a machine that old is still capable of running the "latest and greatest". Then again, we have a PC laptop from about 2005 which is running Windows 10 reasonably well, so....
I upgraded on (2) desktop iMacs.
The only issue I experienced was with the Apple mail client. After both installs the inbox was empty. Per a suggestion on the Apple discussion forum, I ran the Mailbox, Rebuild commands from the menu and the mail became visible again.
While I would have searched for a solution, that would definitely freak me out. Surprised Apple would create an issue for one of their primary apps.
I'm surprised too. This bug has affected a lot of people. Here's a post from the Apple discussion forum about it...
https://discussions.apple.com/thread/7221489?tstart=0
That discussion is about an iOS 9 problem. This thread is about El Capitan.
I like to live on the bleeding edge, especially with "sandbox" computers and when I have good backups.I'll probably wait a few weeks to hear if there is any fallout before I install