The slowest film I've used in mine (modified) was 50 ISO, and I used a tripod (as well as a #25 red filter, since it was Konica 750-infrared film). Results were, expectedly so, fairly blown - but still we got enough of an image to want to experiment with this again using a denser filter combo. Haven't tried it yet...it's on my ever-growing list of things to try.
For general purpose use, I've shot 125 and 400 ISO with no problems.