Now that Apple does public betas, I'm less leery of jumping on the bandwagon within a week or so of official delivery. I've got a 2015 13" MBP and a 2015 15" MBP both running El Cap, including with Adobe CS 4 (!) with no problems (beyond the CS 4 issues that were introduced back with Lion).
The Mail issue just requires a manual rebuild of the mail database (select it from the menu) and all the mailboxes and items show back up the way they're supposed to be.
I've yet to have any issues with the new security model, including with compilers and emulators, other than homebrew, which now requires sudo to do its work. I was expecting OSXFUSE to fall over, but it still appears to be working.
As usual, check
http://roaringapps.com/apps?platform=osx to ensure there's not an app with a problem you can't do without. The new app sounds promising, but until then, check the table against what you've got installed.