I have one and I use it all the time for drawing and doing illustrations as it is as others have said, similar to using a pencil to navigate with. To start with go to a store if you are by a big box chain or an Apple Store and see if they have any on display to play with.
The more money you spend on a given tablet the different options you will get in the long run. A Bamboo is nice to start with and offers a good amount of pen pressure (512 I believe) while an Intuos model will give you (1024). Also the more expensive units come in different sizes from 4x6 to 12x12, wide and standard format (both of which can be "mapped" to your screen size). Also with the Intuos models you can at a later time buy an airbrush "pen" or even other types of pens that have diferent uses such as mimicking a painting brush where as the Bamboo can as far as I know only use one type of pen and the mouse it comes with depending upon the model.
As most people that use them will tell you, it takes time and a good amount of use to become comfortable with using a pen since you can control just about every aspect of it's pressure (a little goes a long way).
So I guess it really amounts to how much you will use it and what you will use it for. The amount of money you want to spend will also determine what you can get but it really amounts to how the tablet and pen feels in your hand and again how much and what you will use it for.
Last once you decide on one there is a tutorial that will help you set it up to start with here:
http://guides.macrumors.com/Setting_Up_A_Wacom_Tablet_Tutorial