S2 IS / S1 IS vs S50 / S60 / S70
I have both the S1 and S2. To answer your question about the S2 focusing in poor light I would say that the AF lamp helps a lot. I have gotten well focused shots with my S2 in dim light with the subject being 5 - 10 feet away. (used flash for these). By the way, I could do almost the same thing with my S1 using hyperfocal techniques (setting focus to 8 feet and zoom at widest value). I still keep this hyperfocal setting as my custom setting with the S2 but don't really find it needed often. This hyperfocal setting allowed me to shoot subjects in near total darkness with my S1, using flash of course. The S2, however, provides an advantage over this hyperfocal setting since I can use the zoom to help frame the subject. This is harder to do with the S1 since the hyperfocal setting depends on the focal length of the lens (and lens opening) and I would need a table of values as well as an estimate of the focal length, something the S1 does not do, except at 10X. The S2 does provide a crude estimate of focal length in the view finder, by the way.
So in short, the AF lamp helps a lot, making the S2 focus at close range in dim light at differing zoom settings. I see far fewer complaints in this forum about the ability of the S2 to focus properly under the conditions I've just described compared to the S1.