It works well and it's an experience more akin to focusing a medium format camera (well, almost). The HoodLoupe is quite bulky but comes with a nice padded case that can be clipped onto a belt or bag strap.
I bought the optional Cinema Strap that can be used to hold the loupe in place. As the name suggests, this is probably very handy for video, but I found it a bit fiddly and the rubber bands frequently got in the way of camera controls. Also, I managed to leave Liveview on for quite a while without realising, which apart from any risk of damage to your camera due to the sensor getting hot, drains your battery pretty quickly.
My only real gripe with the HoodLoupe 3 is the price - £75 from Warehouse Express.