May I substitute very ripe banana for all or part of the sugar? If so, what other adjustments should I make?

I haven't done this fruit-for-sugar substitution, but this blogger says 1 cup mashed bananas with a couple tbsp water can substitute for 1 cup sugar.
Have a look:
http://www.organicauthority.com/eco-chic-table/using-fruits-to-replace-sugar-in-your-recipes.html