Bullet bill

From The DS Ultimate Wiki

This crazy eyed armed missile is litteraly alive. It is a great help in Mario Kart but freaking annoying in Super Mario Bros main series games.

In the 2D games, you might find cannons with a skull icon, which are called Bullet Blasters, these shoot Bullet Bills at Mario when he is nearby. The launchers in Super Mario 64 on the other hand are large grey type boxes with a hole, which look very different to the traditional launcher.

Bullet bill can be GIGANTIC in size in some new super mario bros levels...

In Mario Kart DS and Wii they are most likely to be your most wanted item. It makes you go at an incredible speed and knock karts out of your way. You can get him mostly in 6th, 7th and 8th place or if you are lucky in 5th place.

WARNING, still try to guide Bullet Bill in case of crashes/falling off the track and losing precious time.

So in one way this guy is neither good neither bad...but do not raise your hopes and go out there and get a Bullet Bill.