An amazing piece of tech in modern photography is the ability to have wireless automatic TTL exposure control of multiple strobes. The fact that the two flashes can communicate via pulses of light and automatically determine their exposure so fast that you effectively can't tell the difference between their communication and the exposure burst in incredible.
It's one of the most telling signs for me of exactly how fast computer chips can work and I'm sure the chips that make it work are an order of magnitude less powerful than what's in everyday personal computers.
Right now, I've got two Canon flashes that I use in this setup with the master on the camera and a slave setup somewhere else in the room. One of the few drawbacks of the system is that when a flash is set to slave mode its auto-focus assist lamp blinks every second or so. This light is surprisingly bright, especially if it keeps hitting you in the eye from less than a few yards. It also creates an annoying blink on your subject if you happen to have it pointed directly at whatever you are shooting. You can pivot the flash head to try and get this to aim somewhere else, but in general it seems to always end up being a distraction.
The good news is that it appears that you can cover the AF assist lamp and not affect the wireless operation.
It turns out that the wireless sensor is directly above the Canon mark. Also, to give you an idea of how bright the assist lamp is, the blue tape you see is actually two pieces of painters masking tape with a piece of black paper in between and if you look directly at the strobe you can still see the red blinking through it.
It takes a little getting used to and the user interface for controlling the strobes leaves a lot to be desired, but if you spend a little time to get through the learning curve it's a great tool to add to the box.