This is sort of an "impressions after a month of use" by a Mavericks user who finally bit the bullet and waded into El Capitan (10.11.4). I did a fully-clean reinstall of everything, including all system settings, so there's no scruft carrying forward from earlier OS versions, and I don't run a lot of 3rd party stuff, Tunnelblick being the main offender there. I'm running nothing now that I wasn't under Mavericks. Anyway, my primary impression is that the system is almost terminally laggy, both when I'm running in the GUI and when I'm running processing codes from the command line. The laggyness in the GUI ranges from slow response when doing things like moving mail from one mailbox to another to OMG the spinning beachball of death. I've got a 2011 27" iMac (2.7 GHz 4 core) with 20GB memory and a 240 GB SSD (OWC Electra 6G), and while performance under Mavericks was not what I'd like, it's noticeably worse under El Capitan, and I'm starting to consider a fallback (third one, actually) to Mavericks. Something is really not right with things since Mavericks at least, and IMHO since Snow Leopard. Some of the new software technologies they've put into the OS are clearly problematic, and continuing to put more coats of different colored paint (shaded buttons to flat buttons, for example) doesn't fix the underlying issues. Come on, Apple, you may be focused on mobile devices but apps are still being developed on computers, even if they're just laptops. You still need OS X.