After a little practice, the mouse is very easy to use. However, if you have never used a mouse before, you will have to "mousersize" to get used to how the mouse moves and works.
Some people argue that the mouse actually slows you down. Why? You must remove your hand from your keyboard every time you want to use the mouse. If you are doing a lot of typing, this can slow you down.
The mouse does require additional desk real estate. You must set aside space so you can move the mouse around. If your PC is in tight quarters you might not have enough space to make the mouse useful.
Despite these drawbacks, the mouse continues to be the pointing device of choice among PC users.