Through Google Analytics you can see how individual ads and keywords are performing with respect to conversions. (e.g. track how many downloads)
But in our kind of business where sales can come months later, it is not anywhere near surefire way of determining effectiveness. However, personally, my sales drop if I stop using Adwords so for me, it's worth the money. However, it can be a little tricky to know exactly how much you should be willing to pay

Adwords can also serve anther purpose if you are targeting a new niche. To become #1 known product. In those case I think I would advertise even if it was with a small loss... Just to make sure all bloggers etc. would find my product when doing writeups etc.