I like the size of that, but there is no mention of how much charge you will get out of it. My guess is with something that small it will take a while for you to be able to make a phone call if you have a completely dead battery.
I know this has been posted on this site before. Here's a do it yourself solar charger. It costs less to make and you'll get a lot more juice out of it.
http://www.metacafe.com/watch/800000..._easy_to_make/