Magic Lantern is nothing short of incredible. We are using the 600D/T3i for live streaming with a Teradek Cube, and the ClearScreen option has solved most of our problems.
When I set ClearScreen to always, and Half-press shutter to Every second, we get occasional bleed-throughs of the flashing "HS" on the upper right corner of the outputted video. Somehow ClearScreen is not blocking the overlay of the HS effectively enough. I don't know if this is an issue on other cameras, but it is on our T3i.
Occasionally, by shutting ClearScreen off and back on again, especially after we connect the HDMI out and leave it in (configuring via the monitor) we get it to stop. But mostly it bleeds through.