The only places I see that new style mega-menu apparently "working" at all is for a major brand - and I still maintain they are not a good approach for SEO or for Information Architecture. They are a lazy resort for companies that worry more about their own internal use of the website than attracting new outside visitors.
Really, they are often taking the structure of a brick and mortar store and forcing it to fit a web site, whether it is good for the customer or not. And it could be that google is rewarding this inefficiency with the boost it has given big brands.
Think about this: The reason megastores exist in the physical world is because of the convenience of being a one-stop destination. That's great for people who need to drive to get what they want.
But when it comes to the internet, why should you need to visit a web-based mega office supply store, for example, if you are looking for pencils? It would (probably) be better to send them to a site dedicated to pencils than a site that ranks well because it has a boatload of links to pages for office chairs, digital cameras, mp3 players, etc, that are completely unrelated to their pencils section.
Really, the advantage that a diverse product base provides to the customers of a brick and mortar store is more of a drawback for the virtual customer.
Maybe google is loving the crowd sourcing that is on these sites in the form of user reviews, which must mean lots of fresh, unique content.
But how many user reviews of a number 2 pencil equals the same value of the review of an expert in ergonomics, for example?
Maybe google is loving the user metrics, like the click through rate. But CTR doesn't really measure how good a site is; it measures how POPULAR a site is. If the name brand site is well known, it will get clicks (and people will buy from them), because people love the familiar and fear the unfamiliar.