I heard ISO 25 somewhere, too. I shot a whole roll at ISO 25 with an R72 and got nothing but blank film.
I did try a couple of shots at ISO 100 without a filter, and those turned out perfectly.
I'm going to try ISO 2 next time with the R72--I like to shoot with larger apertures and shorter shutter speeds. I think some have reported lower ISOs because of reciprocity failure.