The current site I'm working on typically has 3 levels on navigation. Site-map and navigation has already been done. They have chosen to use a mega menu to show top level navigation items and sub-categories.I'm trying to figure out if this is the best approach. When is it best to use a mega menu? What are the advantages and disadvantages of adopting a mega menu?
You have some great answers. While trying not to repeat the answers already given, the key advantages and disadvantages of mega menus are :
I also recommend looking at Jacob Nielson's article on Mega menus which has this to say
Looking at the example above, though the Mega Menu does provide all the options to the users,it suddenly become like a large sitemap which requires the user to scan around trying to find the content he is looking for
I recommend looking at this article for best practices on designing Mega Menus
Also look at this article : 15 good examples of mega drop-down navigation menus
I agree with what JohnGB said but let me add both Amazon and eBay have similar problems presenting and navigating extremely large catalogs, but they also have tremendous resources. Mega menus can be tricky and can require testing and iteration to make usable, there's often more to them than is immediately apparent. Here's an article about someone gleaning one of the features from Amazon's menu:
and general issues with mega menus:
So a disadvantage or caveat is that they can require much resources to do them right.
A mega menu is useful when you have a large selection of items in many clearly defined categories and sub-categories. Take Amazon or eBay as examples. This would not be a good choice where your categories and sub-categories were not clearly defined for your users.
Their advantage is that they allow you to include a lot of categories and help users to quickly drill down to the sub-category that they are looking for, while being relatively space efficient.
Their main disadvantage is that they require your users to think in terms of the categories that you have selected, and that they only reveal subcategories when you select (or hover over) a category. However in many situations, there is no better option.