El Capitan was released a few days ago, and I want to upgrade. Will there be issues with the software, or will it run nicely on El Capitan?
My Mac: Retina MacBook Pro 13", Early 2015
I had the same error message as Stefan on opening. I fixed the problem by installing Silverlight 5.1.41212.0.
So providing you've got the correct version of Silverlight installed, you should be able to run it.
I've recently upgraded to OS X 10.11 El Capitan (as a matter of fact, I have the exact same model of computer). I've done some basic tests with the EV3 software without running into any issues. I've downloaded programs to the brick using both USB and Bluetooth.
Base on past experience with OS upgrades with both the NXT and EV3 software, the problems have usually been with Bluetooth. The fact that this is working is a good sign.
I have not used the EV3 software extensively yet on OS X 10.11, so there could be some minor issues I don't know about, but at least I can say that it is usable.
The official LEGO.com page says:
Mac OS 10.6, 10.7 and 10.8 with the latest Service packs
This does not include 10.11, but seeing that another official LEGO.com page says:
Fixed Bluetooth crashing in various ways on OSX Yosemite (10.10)
it seems probable that LEGO supports their software better than their homepage :)
So while we don't have a definitive answer yet, I think even if there are show-stopper changes in El Capitan, LEGO will patch the EV3 to work flawlessly.