I've noted this new camera mode a few weeks back and never really used it, it appeared way buggy. But i checked it today and it was quite useful. There are a few glitches that Valve need to iron out but wholely, the time it could save between compiles seems significant. It seems to have a problem with horizontal displacements (which are 100% illuminated, regardless) and also brushes don't block light, only recieve it (but understandable considering this isn't actually compiled yet). So light tends to leak through walls. Also the skybox doesn't emit light and light brightness is of course only estimated. But really this seems quite helpful for me getting my lighting completed without compiling so frequently to 2nd check everything. Obviously i'll need to compile to get the lighting correct for sure, but this lets me confidently set up lights in one go and go from there. You'll find it in your camera mode list..