The Wacom tablets are USB connected, and the Intuos line so old I'm sure it is USB 1 or 2. However the weird thing is (as BlueSpud points out) the tablet is registering OK in the USB system (you can see it in a System report) and seems to be aware of the pen stylus. On my system there is no tool / pointer drawn on screen, but OS X appears to be responding to 'edge' events (such as hot corners on desktop) via movements in stylus - but nothing at all with the Mouse.
I guess this is probably a Wacom driver issue rather than OS X 10.11 - but very helpful to know that others are seeing it too.