Manufacturers would like us to believe that sugar alcohols have very little affect on blood sugar, but in fact it depends a lot on which sugar alcohol is in the product. The chart at the bottom of this article about sugar alcohols has a lot of information about this. Note especially that many of the sugar alcohols aren't as sweet as sugar, so more must be used to get the same sweetness. Also note that many sugar alcohols (most notoriously maltitol) can cause gas and other adverse intestinal reactions.
Erythritol is the only commonly-used sugar alcohol that I feel comfortable recommending not counting in the total carb count. I don't eat maltitol at all, but it could be counted as having 3/4 of the stated carbs. Sorbitol can be counted as half of the stated carbs -- and so on, according to the chart.
