One point to be aware of here is something that other parents have pointed out, and that's that this particular set only includes one pair of legs, which means you'll only be able to craft one hero at a time - unless or until you have additional sets. Should you buy this as a first set, I'd recommend picking up at least one additional hero (usually less than $10) to keep the action rolling for hours of fun. (pictured on my site mixed with our existing figures)
Here's what's in the box:
Official Description (provided): "Build the ultimate Avengers mash-ups with this special edition Ultimate Avengers Set! With the heads, weapons, and other parts in this set, you can put an Iron Man head on a Captain America body and give him a Hulk arm! Then you can arm him with a Thor hammer or other weapons! Swap parts from other Super Hero Mashers figures (sold separately) to create even more heroic mash-ups. The options are many, so action up your Mash-Ups with the Ultimate Avengers Set! Marvel products are produced by Hasbro under license from Marvel Characters B.V." -- Available at Amazon and other retailers Nationwide.