I used Cactus V6 ii with my K-5 and it works like a dream.
First, it gave me a scare when the camera took about a second or so to shoot after I press the shutter release (flash didn't fire either). However, when i set the flash mode to 'flash on', it stared firing the flashes at every shutter speed (initially the flash mode had been set to 'flash + red eye reduction').
However, when I mounted my Pentax-M 50mm F1.4 on the camera, the shutter speed was limited to 1/180 sec. The camera does not allow faster shutter speed with the manual lens.
Hope this helps someone.