In reply to Tom Hutton:
Great. You grinding your own beans with it?
The thing with the Aeropress is it is open to experimentation and personal taste so you can play with the types of bean, how fine a grind you use, the temperature of the water, and how long you let the grinds steep before plunging to get different flavour profiles.
With the grind I use I pour in just enough water to soak the grinds at 90C for 30 seconds and allow the coffee to "bloom". Blooming is the gas being given off on really fresh coffee which makes it foam up so adding some water first for 30 seconds, and then topping up, stirring for 10 seconds, steeping for 30 seconds, another quick stir, and then plunge for about 30 seconds.
If it is really hard to plunge, your coffee is ground too fine or the water is too cold.
If it plunges too quickly the grind is probably too course.
If it is bitter your water is probably too hot (90C - Or about a minute off the boil with 1L in the kettle for me in the office).
If it is weak and watery then there hasen't been enough extraction time. As the grind gets courser you need to steep for longer.
If the coffee is really tar like and tastes bland then it is probably over extracted. Reduce the steep time.
Rinse your filter paper with hot water first before using it.