I have a Wacom Intuos tablet and I find it a wonderful tool. When I first got it I thought that I would not need a mouse anymore, but it turns out like Doc said that you end up using the mouse for things like menu selections, general large area painting, and things that require more broad, courser movements.
The pen, on the other hand, is much nicer for more specific, finer type movements like drawing in eyelashes, adding back hair from where it got removed during an Extract type operation, and anytime that you need to draw in a freehand line. One of the tricks is to use the different pen styles that vary with pressure. Some pen styles vary the width of the line with varying pressure, and this is nice for things like eyelashes, and other pen styles vary the opacity with pen pressure, and this is nice for doing things like lightening the areas under the eyes.
Not that I'm any good at it, but when the pen is set up for airbrush you can even get a different pattern based upon the tilt of the pen while you are airbrushing something in.
I think with a little time and use that you will start finding quite a few areas that you will use the pen, and that the mouse will feel clumsy to you then.