The Vibration pack for the N64 does exactly what it sounds like it does. It vibrates. For games such as Goldeneye and Perfect Dark, the pack vibrates every time you shoot your gun or are shot. Hooray. Why do you want to vibrate when you get shot? For games such as Diddy Kong Racing, the pack vibrates when you collide with someone or with something. Why? When inserted into the back of your controller, it increases the weight needing held in your hands significantly. The controller is now VERY heavy, and the only bearable way to hold it is to rest the controller on the floor while you control it with your hands. The batteries need replacing frequently, otherwise the vibration will feel more like a small shudder.
However, the 1MB memory is another story. This is incredibly useful in some games as it means you can save your games at certain points. You should always check the box of the game you are about to buy to check if it needs extra memory in order to save your game, as otherwise, you won't have much fun. Many games use up all the memory on the cartridge, so there is no room to save games on to it. Hence the need for extra memory in the controller.
This product is good if the price is right, but if it isn't, I'd stick with a simple memory pack for the controller.