I've had a chance to play a few sessions of Mighty-Six with my playgroup, and I must say - I'm in love.
I'm fairly new to the D6 system, but it is fast becoming my favorite. Mighty-Six takes an already great die-system and adds a lot of flavor to it. Combinations for PCs are virtually endless, allowing you to customize the super-hero (or villain) of your dreams. My GM is a particularly clever fellow, and manages to mix humour into the campaign. We've had some pretty hilarious nemesis's (nemeses?) crop up during our routine super-heroing.
My only criticism is that some of the game mechanics require you to pop in and out of the book for reference purposes, but that's pretty common with intricate game-worlds and systems. This criticism is also partly assuaged by the fact that the art is top notch. There are some really clever "side-bar" images that evoke the glory years of 1980's comics books. Definitely made me smile.
I'll post an update once I've gotten the chance to play Mighty-Six some more, but as of now, myself and my group are really pleased with the product. I would highly recommend it, even to people who don't typically play D6. It's pretty easy to pick up.
[5 of 5 Stars!]