Portabello mushrooms are the strongest of the cultivated mushrooms. It is often used in place of meat as they are very large, almost patty shaped, and take to the grill. With a simple garlic, and thyme marinade, you think you were eating a steak, they are so meaty in flavor. 

Portabellos are low in sodium, and high in potassium, and are low in calories, and good for heart smart eating. One cup has only 20 calories. 

For all types of mushrooms, look for clean caps, free from blemishes, slimy spots or signs of decay. Allow for 1/4 to 1/2 pound per person per serving when used as a vegetable.

Store mushrooms, unwashed in a plastic or paper bags. Keep away from strawberries, and other vegetables in the produce refrigerator, as the spores can hasten the destruction of these fruits and vegetables.

Wipe with a damp cloth before use. Do not soak mushrooms.

Mushrooms can be steamed, broiled, baked, fried, or grilled.

Here are some great resources for all things mushroom:
http://www.mushroomcouncil.com/varieties/portobello.html
http://www.melissas.com/rtf/drportmu.htm
http://www.americanmushroominst.org/