I agree - ratings vary significantly from brand to brand. (For example, the Ventra and Mitylite I mentioned are both rated as 40-degree bags by their manufacturers. However, based on my experience with both, I'd put the Ventra at 45, and the Mitylite at 35 - not quite comparable.

However, within a single manufacturer's line of bags, you can at least rely on the ratings to give you an idea of how warm one bag is versus another. (For example, my WM Mitylite is about 10 degrees warmer than my Megalite - and both are about 5 degrees warmer than their 40 and 30 degree ratings, respectively.)