Here is the demo:
http://nettuts.s3.amazonaws.com/819_megamenu/demo/index.html
I used the "4 Columns" cell to hand-build my top level categories when I tested the Supremo template. I suppose more such cells could be built, as needed.
(The new Abbington Mega Menu template has the same drop-down styles a little different from Supremo.)
Other than that, these mega menus so far use the standard zencart drop down: a single line of top level cats with subcats flying out from the side. It may not work or look that great with a large number categories, as seen on the link in post no. 48 in this thread: allforyourwedding com
http://www.zen-cart.com/showthread.php?194737-Abbiamo-Supremo-Support-Thread/page5
So while you have an impressive-at-first looking horiz drop down, the actual categories are pretty contained in a smallish area. But some handbuilding could overcome this, but maybe not if you have many categories.
How many categories will you have? Figuring out the best nav layout is always a concern and challenge.
There is a version of the zencart drop down menu where you have a choice: 1. standard drop down in a single line with side cats flying out; or 2. Top level cats across the top with subcats dropping down from them. The Blue Stripe Basic template comes with this choice (it's messed-up at zencartinfo) but had a couple of other issues. But was an overall nice template, I thought...liked it alot when I tested it.
But any horizontal category navigation is limited by the width of the site...unless you're willing to have them wrap to two or more rows. Unlike side nav categories, which are theoretically unlimited in to how many you can have.
You could also consider using header EZpages across the top linking to your most important categories, and use standard side categories for the bulk.
Or use the default zencart top categories, which will wrap as necessary.
I understand you have a ton of products. Do you also have a ton of categories?