I'm sure he'll answer shortly but IIRC when I did my hood which was about 68 Lbs bone stock, it ended up being only 18 Lbs more or less taken off. This was of course way before I went Aluminum on the whole thing which took off a lot more.
In general, the undersides do not garner too much to break open the champagne, where as the real weight is in the flat lands panels of either the hood or deck lid. Every ounce counts as they say and its high and up front on the car.
The deck lid undersides on the other hand takes off little since its less real estate of skeletal structure. It is fun though and most importantly, Free. I'd do it if I were on a budget.