Thanks, Juro; this is bound to be useful.
I took up crunches to reduce my middle-age spread. I worked up to doing around 110-120, and have done as many as 200. I do 30-50 until I'm out of breath, read a magazine for a minute or two, then resume.
Bad news: crunches did nothing to reduce my waist size. My lower abdominals are much firmer, but the belly fat overlay remains. The good news: my frequent backaches (which afflict so many of us) have pretty much disappeared!