The old known graphics issues continue to be present, since Apple brought Metal, which is a new graphics api but didn't do anything about the OpenGL drivers (just as I suspected ). Games run neither faster, nor slower and I haven't found any new graphics issues. The Dolphin interface appears fine with the new default font El Capitan brings.
I wanted to compare how Dolphin runs with El Capitan vs Yosemite in this thread, but there are no important differences to note. But before doing so, don't forget to check if your other apps are going to work properly after the upgrade! It would also be better if you first read other threads or posts about how Dolphin runs in El Capitan, because you never know!
If you want to upgrade, but you worry about just Dolphin, it's most possible that you can go ahead.
If you want a quick answer, it's that Dolphin runs fine in El Capitan (at least for me!). Just yesterday I upgraded from Yosemite to El Capitan and tested Dolphin to see how it performs on my Macbook Pro Retina 13'' late 2013, using the new OS.
