The soccer ball has been an item since Brawl, while far from my favorite, it's a good one. It's actually based on the soccer balls from Super Mario Strikers.
Like the game soccer/football, you can't pick up the soccer ball. But if you hit it with a smash attack, you can make it fly.
Which can be awkward and easy to miss, many a time.
But I've always liked it. Once the soccer ball hits its target, it packs quite a punch.