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I don't think it is worth to buy a game-pad for one game, you can buy one if you have more games to use it.

its easier with a game pad...but like brotbaum mentioned, not really worth it unless you have multiple games that are good for it

I use one whenever I can. And for SMB it's kind of a must, they even tell you at the beginning!

But if you really only play FPS or strategy... you might not need it.

Unless you only play FPS or Strategy I do strongly recommend you to buy one. It's great for GTA-like games, racing games, sports games and that stuff.

I bought the Microsoft XBOX fo PC one, and I'm much more than happy with it, and it's not that expensive.

SMB is perfectly playable with a keyboard, so I would advice against buying a gamepad.

If you are really used to playing with a keyboard, a gamepad isn't necessary for this game.
I own a gamepad, but I finished the game with the keyboard without problems. I tried playing the game with my gamepad because it's supposedly easier, but I'm so used to playing with a keyboard that I quickly came back to it.
However, I own the first version of the Xbox 360 controller with a shitty D-pad so it didn't help for a game requiring such precision in controls I guess, but I doubt it would have been that better even with a better controller...
Anyway, as others have said, it can always be useful to own a controller though, but if you are sure you will never need one for any other game, I don't think it's worth it.