If you bought the Wii U and 3DS versions of Super Smash Bros., you were likely already enjoying the fruits of free Mewtwo DLC for a while now. But starting now, anyone can buy him for $4.99, which will net you the character on both platforms. If you just opt for one platform (I suspect a lot of 3DS-only people), you'll save a dollar.
As someone who still plays Smash fairly regularly on Wii U, I think he's definitely worth picking up. He's not a particularly flashy character nor does he have custom movesets (which still aren't out by the way, despite speculation), but he's a great addition to the core game.