
Decks, they're beautiful and fun to use. Here in San Diego, they also add to the value of your largest investment. Not only are you improving your property, you are adding a usable and flat space to an unusable slope.
Here's a great article on MSN online that reiterates the value of adding a deck to your house, especially in a down economy.
http://realestate.msn.com/article.aspx?cp-documentid=16613127
The value of your investment depends on many things. Here in San Diego, most likely 100% or more of the cost of your deck will be added to the value of your house. I read that in some instances up to 110% can be expected.