I have one of these. I have never used it. The last time I took my Hennessey Hammock and my JRB quilt on the Long Trail in Vermont around this time of year (2 years ago) I didn't have one. Yet though it rained for 8 days straight, my quilt was fine -- due in large part to the MacCat deluxe tarp I used.
Now I'm going again. I'm debating whether or not to take it the shield. I have both the under shield and ...um... "over" shield.
Of those of you who have them, when do you use them? When do you feel you NEED them?